Frankly, I find it difficult to believe that the power transformer started buzzing because of a low voltage situation. A properly designed transformer shouldn't care one bit what the input voltage is.

Perhaps a more likely explanation was that the heater was causing distortion of the AC waveform, which certainly could cause a transformer to buzz. Alternately, the DC regulation stages (if any), could have dropped out of regulation from a voltage sag, causing the buzz.

Whatever the cause, your issue is solved.

All I'm saying is this is what Scott said. I also went and put the Fluke on the circuit and found 118v present w/o the space heater and no buzzing amps. I switched on the space heater and watched the voltage drop to 112 and heard the amps starting to buzz.
Call it whatever you like. Bottom line- low line voltage caused the amp transformer to buzz. Strange but true.
Thanks for all the advice. I hope this doesn't happen to anyone else! But, if it does, now you know.
Rob

Hmm- magnetic field ain't coming from tha amps. The buzzing is long gone since they are on a new circuit, but the magnetic distortion is back after a few days. Fixed one problems, though!