Mr. Bill wrote:If the magnet got hot, I'd investigate the possibility that the amp is going into high frequency oscillation. If the output wires are too close to an input line, etc. the amp can start to oscillate at frequencies too high to hear, but not too high for the amp to reproduce. This could also cause the distortion that you are getting.
No problem with the distortion anymore since I've fixed the mic lead, but I'll investigate your other suggestion.
There was something unusual going on, but it'll tale a while to pin down.
Unfortunately I can't test this stuff when my family is home.
For some reason my dear lady doesn't enjoy the experimental process.