Even if there were no more than 8 Parts, were they all located in Part locations 1 through 8 (nothing in 9-16)?

A word on seamlessly switching sounds… often misunderstood. AnotherScott is correct about the Parts and their locations. SSS is a way in which the MONTAGE/MODX can move between Performances without the sound being interrupted — the goal is the listener would be unaware that the performer had stopped playing one set of sounds and are now playing another set of sounds. No cutoff in sound.

The MONTAGE can transition between two Performances each with a maximum of eight Parts, located in Part slots 1-8 (with Parts 9-16 empty).
The MODX can do this with a maximum of four Parts, (with Parts 9-16 empty).

When you setup a Multi Part Performance, each of the AWM2 oscillators in a Part, and the sum of FM-X Carriers of a Part, can be routed through either one, the other, both or neither of the two Insertion Effects. That’s right, on a per soundmaking source basis, you individually route each through the available Insertion Effect blocks… this is equivalent to a studio patchbay with a ton of spaghetti wiring…patching things through different devices.

When you switch Performances you are unplugging and re-plugging all of those patch cables! When you understand this, you begin to understand why there is an interruption in the sound. The more complex the architecture, the bigger the hiccup. Try plugging a dozen or two dozen parch cables and repatch them all without interrupting the sound! It’s not possible. So just how is it done?

All 16 Parts of the MONTAGE can have two Insertion Effects of their own. So no unplugging and replugging need take place to switch from one 8 Part Multi Part Performance to another. There are enough resources for the sound to be maintained during the transition. The MONTAGE can run all 32 (2 per Part) boutique grade Insertion Effect processors simultaneously.

The MODX can have two Insertion Effects simultaneously active on any 12 Parts. Thus the reduced number of Parts that can be handled in the seamless transition. There must be enough resources to continue sounding while the new data is recalled and begins playing.

But the bigger takeaway here is this: because a Performance’s intricate routing per oscillator tone source, must be maintained to create the seamless sonic transitions, you see why SSS is a snorkeler’s tool. If you need to switch from a piano to a lead sound, and back to a piano during a particular song… you could setup to use SSS, and simply recall the already programmed Performances. Quick - Easy. No special programming necessary.

However, for the scuba diver synth player, as you get to know your instrument, you realize, that if you create one Performance that houses both your piano and your lead sound, you can accomplish the instrument change within the confines of just the one Performance. Seamless transitions within the same Performance accomplishes the task without relying on SSS.

Instrument transitions within a Performance can be handled by direct Part selection, and/or by managing KBD CTRL via the Scene buttons, and/or by morphing via the SuperKnob/Assign Knob system, or by manipulating XA CONTROL, etc., etc.

Relying on SSS allows those who prefer to work with already completed factory programming as separate entities. This has its place, to be sure. But Seamless Sound Switching has its limitations, as stated, and does not account for Arpeggios and Motion Sequences… here's why: Arpeggios, obviously, once you think about it, cannot transition… if one Performance has an Arp assigned and the new one has none assigned, everyone will notice when the Arp abruptly stops (obvious, to most of us but it must be stated for the record), likewise, if you have triggered a Motion Sequence, that is set to run a set length, it may not abruptly stop, it may continue to possibly impact the next selection. It depends on what is programmed…

All the more reason to dig in and address all the components you are transitioning. There is no guarantee that every 4 Part Performance with SSS will seamlessly transition to another 4 Part Performance, on a MODX, or that every 8 Part will transition perfectly into every other 8 Part on a MONTAGE.

In other words, it is possible to create things that don’t transition smoothly. It’s something like not all drum grooves work just because the tempo matches, your mileage can vary. Or like if you set the Release Time on a sound so that it take 90 seconds or more to fade out, it might just hang in the air for while. (It’s simply a choice that can be improved upon…).

There are more ways to smoothly transition within the Performance… that is one of the key design concepts. Example…
XA CONTROL is Expanded Articulation Control, and is design to allow the performing musician to smoothly transition sounds within a single AWM2 instrument Part. It allows you to switch seamlessly between sampled sounds because it does not cutoff the sound, it allows all subsequent notes to trigger a different sample Element instead of cutting off any Element already engaged and still sounding.

Samples typically must be retriggered in order to transition - the MONTAGE/MODX offers a method to switch sound samples during a held note-on… by being able to continuously control the individual output levels of triggered samples, you can morph between those that are sounding, smoothly transitioning between samples. This can be done all the way down at the oscillator level (Element) of the architecture, and even at the entire Part level of the architecture.

This puts transitioning between entire Performances via SSS, in perspective, as something that is Quick and Easy - taking already finished programs and moving between them… if you dive into the synthesis engine (scuba diving goes deeper), you may find new ways to generate the kind of *custom* Performances that actually serve you in what you need to accomplish.

I find it much easier to use scene buttons to switch between virtual performances within a single actual Performance rather than using the touchscreen, buttons with a series of many presses, or pedal (advance to next Performance in only one direction) to accomplish this.

Therefore, if ever there is a situation where during a single tune I want to change sounds - I strive not to split up the sounds to multiple Performances. Sometimes this means selecting Parts that are Single-Part rather than multi although, more frequently, I can usually use multi-Part sounds that I initially intend (and/or delete Parts that are not useful to how I use the multi-Part instrument).

Still, the rules of SSS have been outlined here - so you should know when SSS will work or not and the limitations of SSS even when switching can work.

sorry for late reply. been busy at work (glad though)

Thanks for all the info, anotherscott, badmister and jason. I appreciate it a lot. it gives me a lot of ideas!