I've tried to replay a multi-channel midi with the YM2151 shield. I've imported some OPM instruments, which I've mapped using the java application (instrument 1 to channel 1, instr. 2 to chan. 2 etc)
The problem is I can't play instruments according to the channel choosen, either using a virtual keyboard which can choose which channel to play, or with my midi sequencer. It seems only the latest loaded instrument is played, regardless of the channel used. I've also used the "sync instruments" option, reset the arduino, but it's the same.
Now I'm wondering about this "poly mode" and "set poly instrument", I assumed it was for being able to play several instruments at the same time, but I haven't tried to remove the option yet (I don't have access to the shield at the moment), maybe it's for something else and interfere with the midi...