1. Some filters (e.g. LPF12+HPF12) don't have graphs

Just the LPF side is shown. An LPF is fed into a HPF. So you take this graph and then what frequencies are left over you feed into HPF12 (which does have a graph). It'd probably be better to "sum" the two filters visually on the screen - but you will notice if the LPF cutoff is high enough not to squelch out high frequencies then the HPF12 cutoff part of LPF12+HPF12 sounds just like the HPF12 filter - so the "picture" of the HPF side of this can be gathered by switching out to the HPF filter to see what its cutoff does and then switch back and superimpose. Kind of clumsy, I admit - but still no mystery.

Those same filters only let you change the HPF values - I can't find any for the LPF end of them

HPF cutoff is labeled. There's another "Cutoff" value (with no label) - this is the LPF's cutoff. Resonance is for LPF too - not HPF (HPF doesn't have one for LPF12+HPF12). You know this because you recognize the graph that is there is "obviously" a LPF. At least you can figure this out without too much doing since the shape of the graph matches all of the LPF shapes and is closest to LPF18. You can see just by switching between them that a lower number is a more gentle slope down from the highest part of the graph down to the cutoff point. There's a more technical description - but this general feel type description is fine without getting your slide rule out. At any rate, when you dial in the resonance you see the peak on the LPF where you would expect it matching all of the other LPFs that stand alone (non dual filters).

3. For dual filters and the combination ones you can only change one of the cutoffs and you can't vary the other 'split related' parameters at all.

Above covers LPF12+HPF12. There are two cutoff values available. LPF5+HPF12 is the same.

LPF12+BPF6 shows you everything. Cutoff moves the LPF just like the alone LPF12 and then "distance" is an offset from the LPF12's cutoff ("cutoff" parameter) where the BPF6's cutoff is placed. So BPF's cutoff will move along side the LPF's cutoff value (when you adjust "Cutoff" ) and then how far away BPF is placed is set by "distance" which is actually just adding (or subtracting) the BPF cutoff value from the LPF's.

Higher level cutoff will offset cutoff. So if you turn the dial on the "cutoff" parameter then this is what the higher level cutoff will do in its offset. By playing the knobs you can see that the common offset knob will only change "Cutoff" and not "HPF Cutoff" for the LPF+HPF combos. You can even get a sense of how far the offset takes you by moving the common cutoff knob through its travel and see where the LPF's cutoff is placed for the minimum (maximum subtraction) and maximum (maximum addition) points. If the LPF's cutoff is set to 255 then anything past "12 o'clock" of the common cutoff knob (clockwise from here) will not add anything or change the cutoff. 255 is the ceiling. The half turn from 12 o'clock to full counter clockwise doesn't take your cutoff to zero. It sounds like 127. Which makes sense. If you set the cutoff to 127 then your offset knob would allow for subtracting 127 and adding "128" which brings the cutoff to all of its values. If you bias the cutoff (set the parameter) to a value closer to 0 or closer to 127 then you also bias the "effectiveness" of the knob since a value near 255 won't be able to go to zero and a value biased near 0 won't be able to offset (with the knob) to 255. However - this could be a good thing if you want to favor one "side" of the possible cutoff values while using the common cutoff knob. Otherwise, offsetting with something like motion control (or motion sequence) can apply bigger offsets and yank 255 down to 0 or push 0 up to 255.

As much as could be improved with the instrument with respect to filter visualization - it's still fairly far from a mystery.

A more pronounced mystery is the math behind offset values when applying various curves. Of course you could spend the time to discover the math since pitch is particularly revealing. However, it would be better to be able to "see" what the offsets actually are to more quickly determine if your programming is in bounds. And to the complement the "by the ear" approach which is valid but shouldn't be exclusive.

There's also downright wacky stuff where it seems like parameters are intentionally presented in a confusing manner. One learns to deal with these (since it's engrained in the DNA of all past Yamaha synths too -- learning will serve you later if you get Yamaha in the future). But those mysteries (initially) are stumbling blocks I just don't see much of in the way of how filters are presented.

The treatment and presentation of filters is, for the most part, fairly conventional.

@Antony

Yes, I do it this way ... project by project ... feature by feature. But one thing will never happen... that I "jump to ceiling", because this machine is so fantastic. Never ever 😉
The sound? ABSOLUTELY! AWESOME!
But long time ago I played a Wersi Organ ... WOW!!! WOOOOW! Action, action, action. Nothing for whining piano-players ... no weights on the keys ... no loud hammering keybed ... nevertheless WITH velocity and aftertouch (mainly for vibrato). Cool thing. Unfortunately: now 23.000 bugs. Times have changes, the market got very small, the prices high.

The MODX. I simply don't have that fun, I should have. That is the main reason, why my MODX catches dust, whereas my keyboard and ePiano are sparkling clean 😉

Trouble, trouble, anger and trouble ... with every single feature. The least problems I had with MS. Only with those 9 combinations I have to try out in order to fix it right. 9 trials is acceptable 😀 Goes fast, while playing.

The only reason to calm down: cheap, beginner-model ... it is like it is. For this price you won't get something camparable.

Learning? Recall...

- I am a passionate PLAYER ... not a physician or designer. And playing will allways be my fun. Programming is more or less a required thing for me. I bought a synth because of those "unearthly" sounds. And they are!!! Yamaha allways had the best sounds (my keyboards) ... my opinion.
But I didn't know, that the MODX will cause so much trouble.

- I do this as a HOBBY. Means, I don't have hours of hours each day for the MODX. I nearly use to play every day; but for fun/relaxation. And not the MODX, because "work-station" is the right word for this. 3:30 minutes of fun costs weeks of work.

- I am OLD (50+). And believe me or not: the age makes a growing difference;) At least with me.

- And perhaps I am not intelligent enough. Allthough I am very interested. But I nearly understand nothing of some instructions published by Yamaha-people. Starting with Adam & Eve ... left, right, back, forth ... MIDI-theory in between ... a bit of quantum physics for the synth-freaks/pros/semi-pros. Huuuuuhhhh
In addition english is not my native language. I learned british ... and no technical terms. Google, google, google. I have serious problems with those written discourses. Much like with this Operation System. Chaos.

I need ... this effects for this result/sound ... do this, that, here, there ... finish.
In this forum I get this from many users. Fantastic!!!

Take a look at german "tech talk" ... he allways starts with "we have no time - we have no time - we have no time". So what!? Perhaps they should call it machine-gun-feature-demonstration. And sorry ... to stop and repeat every 5 seconds??? Nope.

First I found some nice users in YT. Very rare!!! In the meanwhile I found Moessieurs (french) and Sigalas. Much better! Fine for my IQ:D Do this, do that...heard how it sounds... cool
For constructional inspiration I now watch DAW videos, where people explain how they made a song, the sounds, the effects. That is really practical approach for me to deal with this.

Hard enough to study all those things. But no matter which feature I apply ... problems ... deficits ... isufficiencies. "Fun" is something different. I only play the MODX once in 2 days or so.

And don't ask me for the many faults I found. The latest was "6 ...guitar" or "steel guitar" ... I don't remember the exact name. Volume control not possible. Could be from the expansion package. I call this: Section "MODX special" ... other things I call "well sounding errors" 😉 My list of faults is growing each time I play with it 😀 But who cares ...!?