I have a 770 driving 7 x 6-ohm loads, which I have listened to briefly at extreme volumes in the 7-stereo mode of the 950. This demands more power over all and feeds much more power to the surrounds than almost any surround sound source. No problems with distortion onset as higher levels are reached with all sub crossovers set to 80Hz. Not a large space but got the ol’ RS SPL meter over 120dB for a little while. Admittedly, most of the time the needle got that high was during moments of heavy bass, but the other speakers were still giving off sound which gave me mildly ringing ears. (I certainly do not do that but once every other year or so – I’ve got to maintain decent hearing for work and continued audio enjoyment.) The 770 got warmer, but not overly hot to the touch. I have almost a foot of clearance over the 770 for warm air to float away.

I agree that there are some simple tests one can do to check other problems as others have suggested. Another test might be to feed each of your three speaker pairs one pair at a time with a well known, simple stereo signal, see how each pair performs on their own at various volume levels. You may have to power down and swap a few cables to perform each part of the test since you don’t want to use processed surround signals for this test. With this test you know you are not near to exceeding the total power supply capacity of the 770 as you slowly raise the volume to high levels on just two channels at a time.

Another simple test might be to use a plain meter to monitor the line voltage supplying the 770 – see if there is more than a volt or two drop during moments of seven-channel high output.

One more test, then I’ll quit: use the same surround sound set-up and source material that lends itself to providing the distortion you can easily hear. Using the same part of one scene again and again, listen to only the center channel, then only the front L & R, then only the side surrounds, then only the rear surrounds. See if any set of speakers will tend to produce distorted sound whether played moderately or loudly. If a set of speakers are “clean” during the “simple stereo” test but “distorting” during a surround test, it may have something to do with the source or the surround processing, not the amplifier or power situation.

I hope you can track down and solve the problem whatever its source.

Oh, if you are hearing “hard clipping,” the sound will be distinctly crackling during the highest moments of the signal. Of course you may be hearing a type of distortion that is not clipping.