freem:
The YM2610 only has native loop support for ADPCM-B; any ADPCM-A looping has to be done in software/Z80 code. (This is typically achieved by having loop point data somewhere in the M1 file, then watching Status 1 (Port 06) for the relevant channel to end playback, and finally set the new playback/ending points from the loop data in ROM.)

please, can you post some example code how to do this, my z80-knowledge is a bit rusty.