I do not own an M200 (I chose 1 x 770 instead of 7 x M200) but if I understand correctly, 'music mode' is looking for an incoming signal in order to turn on or stay on.

Have you tried connecting the M200 to the loudspeaker without anything connected to the input while in the music mode? Does it either turn on or stay on 'endlessly' with zero input? If so, likely it’s time to trade that M200 in for a replacement.

If it does turn off with no input connected, but doesn't turn off when connected to the preamp, there may be signal at an inaudible frequency due to some stray RF, or even a minor DC level difference due to a very minor ground loop problem that does not necessarily noticeably interfere with audible frequencies.

Hopefully we’ll read soon that the problem was remedied.