The same thing happened tonight, only this time I was ready. It appears to be thermal, it takes about 30 mins to an 1 hour (wasn't really timing it) until the problem manifests itself.
With the speakers on the A terminals and the scope on B, I could swap between looking at the waveform with no load, and with the speaker load.
30Hz test tone without the speakers connected.
30Hz test tone with the speakers connected.
60Hz test tone without the speakers connected.
60Hz test tone with the speakers connected.
1KHz test tone without the speakers connected.
1KHz test tone with the speakers connected.
Music with speakers connected. Notice how the left channel waveform looks weird.
I tried swapping the speaker cables from one speaker to the other. I also tried swapping the scope cables, and also the connections at the amp. It's the left channel, and switching between normal and reverse doesn't change, so that sort of rules out the preamp.
Lee.