Hey all, I have a recently purchased B&K 507S2 and after about two weeks, after I would turn off the receiver, it caused 1 rear surround speaker to hum. It doesn't matter if the receiver is even plugged in! I put a voltmeter on the speaker output and measured initially around 2 volts, but it drops slowly. To me, it looks like a capacitive discharge. Has anyone ever heard of this before?
My guess would be you are on the right track. I would assume the BK has a muting circuit which should shut off output to the speakers to avoid any turn off thumps. If that is the case, the unit is obviously defective and should be checked by a technician.