Hi guys. First post. I've been running a 950/7100 through KEF speakers for nearly two years, very happy with it. Lots of good information to be found here.

I think I know what the original poster is talking about. Some time ago I rewired the crossovers in my mains (KEF Q-30s) to support passive or active bi-amping, with a switch to bypass the passive crossover. Haven't gotten around to the active crossover yet, but for the moment I have it passively bi-amped using the two spare channels of the 7100.

I hear it too. When the amp is turned on, everything works as it should. But after shutdown, about twelve seconds after, there's a faint tone through the tweeters. It's a descending sine wave that starts somewhere in the 1 KHz range, and ends about four seconds later and two octaves lower. It's quite repeatable. The only variation is that if I only have the amp on for a few seconds before I turn it off again, the tone starts lower and doesn't last as long.

Because it's a low voltage, controlled waveform and the passive XO is still there to protect the tweets from direct current, I'm almost positive it isn't hurting anything -- it's just weird.

I've seen a couple of other posters note this behavior in the last two years, but no real explanation. My best guess is that the amp is somehow discarging itself differently after being turned off, now that it has a purely capacitively coupled load?

Oh, and I noticed very little difference between normal and passive bi-amp. If I play a track in mono with one speaker normal, the other passively bi-amped, I can barely hear an improvement in the low- and mid-bass. Probably similar to how a bigger amplifier would sound. If you're concerned, I'd say just go back to single-amp and you won't lose much.

Thanks, all.