Standard GM = patches 0-127
Yamaha Motif GM = patches 1-128
I rarely use the ES to play back midfiles, but I have had the same issues when using it as the sound module for One Man Band software arranger.
I have to use OMB in XG mode as opposed to GM mode, and then it works fine. I think the few XG midifiles I made using my older PSR boards also work fine on the ES, while others using the GM standard do not.
It's a pain but... you can remap the voices and then save as a midifile in the ES. If I recall correctly, just changing the mixer settings isn't enough, as you need to insert midi bank / patch change data into the midifiles tracks, although you could also save mixer "scenes" if you wanted to.
I actually did work on a few midfiles when I had the original Motif, but I did all of the editing in Sonar. For me, it wasn't worth the effort, and my classic Mo had polyphony issues to boot with some of the busier midifiles.
My laptop has the SGM180 soundfont in it ( mine is modified a bit ), which is a very good free standard GM sf. I use it in Live Synth pro which I open as a DX insert in Sonar. It sounds better than the GM voices in the ES, and I no longer have issues with midifiles.
AJ
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AJ