I don't own Polk speakers, so I have no experience with the demands they place on the power amp. My speakers do have similar sensitivity to many of the Polk models and are 4-ohm nominal impedance. My 7700 has exhibited ample headroom and could drive my speakers to an uncomfortably loud listening level, but I do only use five channels with both music and movies at the present time. However, I still have a hard time imagining a 7700 having a problem driving any similar speakers at a bearable level in a typical room. If the 7700 were at fault possibly because it was being driven into clipping or because the power supply was sagging, that would be noticeable any time there was a loud passage at high volume. Your problem doesn't sound like that. I didn't think I had anything to add to this thread, until I read the part about the crossovers having been modified between the time you used 3.1 and 7.1 channel. I assume this modification required some soldering. Are you confident there are no intermittent "bad" solder joints in any of the crossovers? That might cause the problem you are experiencing. If that's the culprit, it should be intermittent and not necessarily repeatable over the same piece of source information, although it's possible some source material is better than another at setting up a vibration frequency that exposes a solder flaw. It's also possible that if the flaw is in only one channel, it might be masked by the other channels at higher volumes and be less noticeable.

Bill