So, is there a way to de-stereoize a PART that is using a stereoizing effect like 'Symphonic' (especially if it's an FM-X PART),

Yes, but with consequences… discussed below.

…but without having to use another Insert effect to achieve this?

Why this restriction on the use of another Insert Effect? They are happy when they are in use! If the use case is build a lead sound, you can enlist additional Insertion Effects assigned to your synth Part(s)… also discussed below.

Thanks for the question, hopefully the discussion will open some new possibilities.

Stereo is an illusion…
Where a sound ‘seems to come from’ is part of what the ear/brain uses in its cataloging and sound recognition/identification process. Effects that use the stereo field as part of their illusion may lose most of that if you then sum them (completely) to mono.

Like taking an Auto Pan effect and narrowing the Left/Right movement by forcing it to mono is often catastrophic to the result once you narrow it beyond a point.

Why the pitch change ‘effect’ does what it does, is partially because of the pitches not emanating from the same pin-point location. One ear engaged 0.01ms before the other is more than enough to “localize” a sound source.

This gives the effect of increasing the number of players. Pitch change plus delay “fools” the ear/brain as to how many sources are producing sound and from what distance - it starts to blur the pin-pointedness of the original singular source… and the ensemble illusion is enhanced.

Detuning just one oscillator, does not have the desired affect alone. Detuning a duplicate of the original in the opposite direction or more change in the same direction, says “there is now an additional sound source”. Without a second source, you don’t get any kind of ensemble sound. The “Symphonic” Effect uses like six taps so it’s rich in the ensemble illusion.

There’s a classic story about Herbie Hancock Rhodes sound on a recording and how when they made a mono single 45rpm version of one of his hits in the 1970s… the Rhodes was originally processed with stereo effects… when they pressed the mono 45 the Rhodes completely disappeared! No Herbie, it was just “…and the Headhunters”.

Taking the Rotary Speaker effect and summing it to mono takes the soul out of the illusion of movement.
The “Vintage’74” is an electric piano with Auto Pan — rich and slow, but played in mono it sounds goofy and nervous (fast) — hardly recognizable as the same sound.

Summing to mono can kill or distort what your going for… that is not an absolute, however, mileage varies; the devil is in the details…I understand you just want to narrow it a bit, I also imagine without killing the thickening effect entirely… before we go there, I would like to ask: what’s the ultimate goal?

You mention not wanting to use an Insertion Effect, as if you have concluded you are limited to only the two Insertion Blocks fixed to the Part. This may not be the case at all. Depends on the ultimate goal…

_Are you performing “live”? Live you need it all to happen at once.
_Are you setting up to record? Recording gives you so many options.
_Not all Insertion Effects are a single processor, many are multi-processors.

_If you need another two Insertion Effect blocks you can add them using MONTAGE’s audio routing capability.

Here’s how to squeeze a stereo effect back into a mono tube… place the “Symphonic” Effect as INS A — then route “A > B” — making INS B an Effect that outputs a mono signal. Or try one that can both widen and narrow the stereophonic panorama…

Use the “Stereophonic Optimizer” (Misc: Insertion Effect) to reverse the process… as INS B; Instead of increasing the stereo-ness or “Width” of the signal you can reduce it… by frequency band - they give you 5-bands. This should do the trick. Only you will know exactly what you’re looking for… but explore this Effect Type (very interesting stuff)!

Programming Tip: Adding an additional 2 Insertion Effects
While I don’t recommend routing MIDI OUT to MIDI IN (never found a use for it, I don’t think you can seriously hurt anything). Audio Routing, Out-to-In, is a completely different animal and highly useful. Similar to Routing signal in a studio patch bay, routing the signal using MONTAGE’s Assignable Outs and A/D In allows you to expand your processing possibilities.

Take the “Part Output”, which is normally assigned to the “Main L/R”, and send it to the “Asgn L/R”. (This Routing takes place post the Insertion Blocks, and 2-Band EQ, but pre (before) the RevSend and VarSend).
With two short 1/4” cables connect the “Asgn L/R” Outputs to the “A/D In” on the MONTAGE back panel. Your synth Part(s) will arrive as a LINE level input, with its own 2 Insertion Effect blocks, and Sends to the Reverb and Variation (System) Effects.

Within your Performance, the “Audio In” is on the upper “Common/Audio” level of the architecture.

Result: you now have 4 Insertion Effect blocks to play with…Assign Knob/Super Knob Controller Assignment is available both for your Synth Part(s) and your A/D In on a per Performance basis.

Hope that helps.

Thanks for the comments and ideas Bad Mister!

Why this restriction on the use of another Insert Effect? They are happy when they are in use! If the use case is build a lead sound, you can enlist additional Insertion Effects assigned to your synth Part(s)… also discussed below.

The main reason is because I'd like to utilize the second Insertion effect as something else like maybe a delay or possibly something that can be brought in via AsgnKnobs or SuperKnob at times. And/or maybe I want to use the new Wave Folder effect to give the sound more edge, etc., etc. Basically I'd like to use “Symphonic” Effect as INS A for some PARTs, but narrow the stereo field, but without using another Insertion effect to achieve this, so that I can have that additional Insertion free for some other effect.
Yes, I'd like to have both Insert Effects used and happy, but for other purposes than narrowing the stereo field, if possible...

You mention not wanting to use an Insertion Effect, as if you have concluded you are limited to only the two Insertion Blocks fixed to the Part. This may not be the case at all. Depends on the ultimate goal…
_Are you performing “live”? Live you need it all to happen at once.
_Are you setting up to record? Recording gives you so many options.
_Not all Insertion Effects are a single processor, many are multi-processors.

"Live"!
While I know that routing & sending the PART Output to the “Asgn L/Rin” instead of the default assigned “Main L/R” allows for basically adding two additional effects to the PART, I would prefer not to do that, as I am planning mostly for performing "live" (to partly answer your question above), whereby I may want to send/separate the Bass & Drums PARTs' Outputs to the “Asgn L/Rin” so they can be mixed differently than most of the other PARTs. I also I may just want to utilize “Asgn L/Rin” as a click track for a drummer to follow in the event I end up with a drummer.

Basically I would like to not use 'Ins B' of the PART for narrowing the stereo field and would prefer not to Output the PART to “Asgn L/Rin”.

Also, it's not just for synth solos. One experiment I was doing recently was to take the "After 1984" Single PART Performance (Van Halen - Jump), and add the "Symphonic" Effect as INS A to thicken it up; however I noticed it didn't thicken it at all. It made the sound spread out to wide (in terms of stereoization) and became distant. That Performance has 4 Elements, so I thought I'd try disabling two of the duplicate elements & Pan everything close to Center on the two active Elements, then add the "Symphonic" Effect to thicken it up, but it did just the opposite, spreading out the thickness, becoming more distant and less like what the original sound was for 'Jump', which is basically mainly a mono sound with some added stereo effects.

Using the “Stereophonic Optimizer” Insertion Effect to reverse the process would work (and I agree, very interesting stuff), but I would prefer if there was another way to achieve this.

Unfortunately some of the effects that I'd like to add don't have the ability to reverse the stereo that the "Symphonic" Effect creates.
It doesn't have to be completely back to Mono, but having it close would help keep the sound centered and less distant/spread for certain sounds.

This is actually a great conversation to have. Utilizing Stereo is what the Montage does best, and when used properly, it can really enhance everything you hear from a Performance. And at the same time, having some stuff Mono with some stereo effects sends is good, as it brings certain instruments/sounds forward/front and center (or maybe panned a bit if desired).
I realize that many waveforms are already Stereo sounds to begin with. However, there are many PARTs &/or Elements of a PART that start with a Mono waveform (or an FM-X PART for example), and we can either stereoize them with certain Insertion Effects, or leave the main PART/Elements as Mono with some stereo effects added (via either Rev & Variation sends, or some Insert Effects that don't necessarily stereoize the main sound Mono, but adding an element of stereo to it).

The main goal of this thread is to explore options to keep a Mono Element waveform or FM-X PART as mainly Mono, and add in some stereo effects without completely stereoizing the main sound itself. Certain Insert effects do this with no control to adjust the stereo field. The "Symphonic" Effect is one of my go to effects for when something doesn't need to be front & center and more Mono like a Snair, Kick, Synth solo, older Synth sounds that are Mono with some stereo effects, etc. At the same time I would like to be able to thicken a Mono waveform or FM-X PART a keep the core sound Mono, and less distant/spread out...
Using the 'Pitch Change' Insert Effect can achieve nearly the exact same thickening as the "Symphonic" Effect, but with control over the Stereo field; However it comes at a cost, as it requires adding the effect to both INS A & B, which is not much different than using the "Symphonic" Effect in INS A and the “Stereophonic Optimizer” effect in INS B to adjust the stereo field. But either one leaves me with no options to add an addition Insert Effect (unless I Output the PART to “Asgn L/Rin”, which I would prefer not to).

"Symphonic" is great for Strings, Pads, and other sounds that are best stereoized, spread out & don't need to be front & center. However I've noticed with other types of sounds that need to be front & center (Mono), "Symphonic" doesn't thicken them up the way they need to be.
Maybe it would be worth adding an Idealscale idea to add a new "Symphonic" type of effect that has some enhancements, such as control over the stereoization!? Maybe even make it a multi-processor effect like some of the others, whereby it can do what it normally does, but with added functionality to both adjust stereo & add something like a delay or panning effects, etc. Or maybe add something entirely new that has never been added as an effect on any synthesizer before..!?

We try to focus on what you can do now. The System Effects (Reverb and Variation) can also be re-tasked as a pseudo-“Insertion Effects” when you are using the engine to build an instrument sound, like a Lead/solo sound. (I wasn’t aware the kick drum snare drum would be using the Asgn L/R Outputs…in this scenario.
…in a situation where you’re sending signal to the Reverb and Variation, you have the ability to route all signal through these processors (insert) by setting the “Dry Level” = 0, and using the SEND to feed signal, turning them into pseudo Insertion Effects.

Common Assign Knobs and the Super Knob can be used to real-time control parameters of the System Effect and Master Effect.
Part Assign Knobs and the Super Knob can be used to real-time control parameters of the Part’s 2 Insertion Effects

You have options to setup the Variation Effect as a pseudo-Insertion for a panning Effect, or simply use it as it is normally, Send/Return, for your Delay. It is totally up to you what you do with the Effect Blocks.

Maybe you can use an external pedal. With "one trick" keyboard devices - like Rhodes or Wurly or ... I would use an external pedal to create synth sounds. Not uncommon to carry pedal(s) for these. With a more swiss-army-knife digital synth like Montage/MODX there may be a place for external pedals as well.

I don't necessarily have a recommendation here - other than when I search I find a discussion:

TC Electronics Mimiq is fantastic for converting a mono synth to stereo. Used it to beef up my Minilogue and get a nice wide unison sound.
It’s essentially a doubler, but the “tightness” control allows you to set how close the doubles are.

It is stereo in, but from what I can tell, with the effect engaged it just ignored one side and doubles up the other. So best on mono signals.

I grab this one out of the bunch because "tightness" may be an adjustment on how wide the effect is. Now this would of course apply to either your entire signal out Main L&R or some partial signal out Assign L&R (with its system/master fx stripped) - so the use is limited if really want to cherry pick one sound out of a bunch that have to go out the same output. However, assuming you can somehow route your "lead" alone when you want this kind of effect - maybe some pedal(s) can achieve what you want.

I guess if this were more Yamaha-friendly I'd look up if Line 6 has something that'd do this.

BTW: the discussion I quoted above is from: https://www.elektronauts.com/t/stereo-widener-pedals/148562/8

... and more searching would probably find better discussions more centered around the ability to narrow the stereo field than widening.

Not that you couldn't do this all in the Montage - or should give up trying. I'm just offering that because Montage has standard audio outputs similar to other electronic instruments that themselves leverage pedals - you also have the same pedals at your disposal for shaping your sound.

We try to focus on what you can do now. The System Effects (Reverb and Variation) can also be re-tasked as a pseudo-“Insertion Effects” when you are using the engine to build an instrument sound, like a Lead/solo sound. (I wasn’t aware the kick drum snare drum would be using the Asgn L/R Outputs…in this scenario.
…in a situation where you’re sending signal to the Reverb and Variation, you have the ability to route all signal through these processors (insert) by setting the “Dry Level” = 0, and using the SEND to feed signal, turning them into pseudo Insertion Effects.

You have options to setup the Variation Effect as a pseudo-Insertion for a panning Effect, or simply use it as it is normally, Send/Return, for your Delay. It is totally up to you what you do with the Effect Blocks.

Yeah, I first saw how the “Dry Level” = 0 was being utilized to turn the Variation effect into a pseudo-Insertion for adding a second "Rotary Speaker 1" effect to 'Organimation' Organ Performances. Pretty cool stuff!

That is definitely an option to keep in my back pocket... 😉

Is there a Variation 'Delay' effect that can be adjusted to make a PART more Mono & less Stereo to a certain degree (...similar to how the “Stereophonic Optimizer” Insertion Effect can)..? If so, I'm thinking I could maybe use Variation as a 'Delay' Send effect for most PARTs (which I often do), and for any PART that I add "Symphonic" + another Insertion effect on (ex. "Wave Folder" ) , I could then use the “Dry Level” = 0 trick to turn the Variation 'Delay' into a pseudo Insertion Effect that makes that PART more Mono, without affecting the other PARTs that I want to remain more Stereoized...!?

Maybe you can use an external pedal. With "one trick" keyboard devices - like Rhodes or Wurly or ... I would use an external pedal to create synth sounds. Not uncommon to carry pedal(s) for these. With a more swiss-army-knife digital synth like Montage/MODX there may be a place for external pedals as well.

TC Electronics Mimiq is fantastic for converting a mono synth to stereo. Used it to beef up my Minilogue and get a nice wide unison sound.
It’s essentially a doubler, but the “tightness” control allows you to set how close the doubles are.
It is stereo in, but from what I can tell, with the effect engaged it just ignored one side and doubles up the other. So best on mono signals.

I grab this one out of the bunch because "tightness" may be an adjustment on how wide the effect is. Now this would of course apply to either your entire signal out Main L&R or some partial signal out Assign L&R (with its system/master fx stripped) - so the use is limited if really want to cherry pick one sound out of a bunch that have to go out the same output. However, assuming you can somehow route your "lead" alone when you want this kind of effect - maybe some pedal(s) can achieve what you want.

BTW: the discussion I quoted above is from: https://www.elektronauts.com/t/stereo-widener-pedals/148562/8

Interesting, I'll definitely check out that pedal as an option ... thanks!