Why does my system now have shrill top end.

Absolutely, I completely agree. I posted before I had completed my thought..

Cables certainly are possible, but the evidence presented points to the electrical system as a distinct possibility as well. It would at least be worth shutting off the branch circuit at the panel to completely rule effects from the kitchen circuit out. Also, double checking the connections in the panel (after shutting off the power) would be warranted as well. I just wanted to put those other possibilities out there as well, to avoid needlessly shipping an amp around the country.

Kosst also offers a reasonably easy test as well.. lots of good ideas for more data points.

If Kosst reference to DMM means digital multi meter it is possible the op does not have a halfway decent one or even know exactly what he should be testing/looking for. If the latter is the case I do hope he will ask for help right here because that is an extremely valid test as well.

Kosst_Amojan 11-30-2017
It’s free and harmless to connect a DMM to the amp outputs and determine if the bias is off.

Uberwaltz 11-30-2017
If Kosst reference to DMM means digital multi meter it is possible the op does not have a halfway decent one or even know exactly what he should be testing/looking for.

I could use some clarification on that myself, Uberwaltz. I’m not sure how a multimeter measurement at the output terminals of an amp can determine if the bias is too low (or too high), the main possibility Kosst had cited being low bias. DC offset can be checked for in that manner, of course, but perhaps Kosst can clarify how such a measurement can determine if bias is too low.

Best regards,
-- Al