Couple more comments - First, I don't agree with the term "GM" voices. To me, I instantly think of GM as in General MIDI. Not the case in the world of Ketron. This isn't what you are looking for I only share it in the event you haven't see it. It shows more about editing a voice on SDxx.
https://youtu.be/be5xI9G_ugcFrom your comments, it sounds as though you would like to go one step below what is offered and change the components that make up what Ketron calls a GM Voice. I'm wondering how much is terminology in the disctussion?
I'm making up this example and didn't really look at the SD90 when I typed this. If you take a Ketron GM voice that is made up of let's say two sounds(voices). Let's say Jazz Organ. On the main screen, that is all you see. However, if you go into edit mode you see that that sound is composed of two other sounds let's just say it's organ1 and organ2. Now, you can change a number of things regarding both of those sounds and how they interact to make up the Jazz Organ sound (GM Voice). How different is that from looking at an edit screen and instead of combining organ1 and organ2 the screen is labeled Part A, Part B, or some other description allowing you to mix those elements together to create a new sound or modify an existing sound?
As you can probably tell - We don't do much voice editing other than something as simple as change reverb level, perhaps a little detuning or chorus effects. With hundreds and hundreds of sounds at our fingertips just don't find a need to change them all that much. But I do know some folks that are very deep into voice creation/editing. What do some of the other manufacturers offer in this area and what do they call the various components of a sound/voice?