PC is exactly how you pick different instruments. MSB/LSB maybe you don't have to send - but you certainly need PC to pick an instrument through MIDI.

I can see I won't be nailing down a single Performance we can both agree upon testing, so I'll just start from the top of PC=0 (MSB/LSB=0) which is Acoustic Grand Piano.

Yes, 96 is longer decay. Now I'll have to dig up a sound canvas or something GM2 to compare. I do have something that came bundled with other software - so I'll check out how that player handles CC72. I'll likely come to your same conclusion - that it's longer on MODX. Then I'll see what can be done. Likely you'll need to do "a lot" to get these in line.

For 0/0/0 (acoustic grand) - if I set the element release time to 15 (from the high 60s default) - then an offset of 96 on CC72 brings the release time (in seconds) down to 2.

The element release isn't the only knob to turn - but it must be changed in the case of this piano. Others may act differently - or you could get lucky and setting all element release times for all PARTs to "15" would do what you want.

Due to the possible differences in solutions - it'd be nice to have some specifics of what you actually use - but I can deal with the lack of participation.

The SysEx address would be:

0x41 ep 0x31

where "ep" is in the range of 0x00 - 0x7F to cover all elements of all PARTs. 0x0-0xF is the PART number (0-16). Then add to this 0x00 for element 1, 0x10 for element 2, 0x20 for element 3, and so on through element 8 add 0x70.

Thank you for your find

Do this SYSEX 0x41 ep 0x31 with value of 15 or 60 Don't change anything to the sound release if the CC72 is at value 64 ?

Still in GM MODE, if you then send a program change, do the value 15 stay there or come back to 60 ?

0x41 ep 0x31 with value of anything other than what's programmed will change the release time when CC72 is 64. We're changing the programmed release time so that the offset release time doesn't go so long. Robbing Peter to pay Paul. You'll have to decide if the release is too short for CC72=64 or if it's OK. If OK - then that's a way out. But I only described PC=0 (Acoustic Grand Piano). There's 126 possible other different solutions - I haven't looked to see if the setting of 15 works for "everything" equally. Most likely not.

Unfortunately, using motion control offsets (Mod/Control->Control Assign) isn't a way out since the release time is not a destination parameter and there is not one that serves as a reasonable proxy for it.

If GM2 playback is what you need - and Montage is NOT GM2 by definition - you may want to get a GM2 VSTi instead of going through all of this for your GM2 work. Roland has the virtual SoundCanvas which is expensive as a PC VSTi - cheaper as an iOS app. There are other GM2 players out there.