I have a pair of M-200s behind a Denon 3805 running as a pre/pro for the left and right mains. When I put the 3805 in standby, or even power it down, the M-200s stay on, even though the switch is set to "music." What is the problem?

How long have you waited? The music mode switch waits a fairly long time after the signal goes away before shutting the amp down - this is to prevent the amp from powering off when the material being played becomes quiet or stops entirely briefly. I'm not sure how long the wait is, but I think it's a few minutes.

thanks gonk ... I started to worry at the half hour point

I have mine set on "music" as well. They seem to be pretty sensitive. They'll come on and off (not necessarily together either) at random times even with my receiver off. I just switch them off when I'm done with the show. Haven't been watching as much. When I have more time, I'll have more money to get a better processor with a trigger.

The main towers they are powering have built in powered subs that do the same thing. Those don't turn on and off in sync with their amp either. I turn the amps off because sometimes they'd emit a buzz.