This is probably a very basic question, but... is there 1 or 8 modwheel channels in a GPO player (so 1 modwheel for every instrument or 1 for EACH)? If I load a violin and a flute into a same player, can I draw modwheel data for both of them (on different channels) or do they get mixed up?
Each midi channel has its own set of controls. The visible mod wheel you see in GPO isn't used for anything more than testing different sounds. Assign each sample in GPO to a unique midi channel. In your sequencer you can write different mod wheel data for each channel, they're all independent.