Why does my system now have shrill top end.

I second what @jea48 said: check, or have your electrician check, the connections in the panel. Not just on the circuit feeding your system, but every connection: hot, neutral and ground for every breaker. You may even consider reseating all the breakers. (Please stay far away from the lines coming in from outside. If you don’t understand what I’m saying, please hire an electrician or ask lots of questions)
I know that loose connections can add significant noise to the electrical system.

Definitely try the amp swap too. Give us more data points!

Good luck!

+1 @grannyring 

Al and (most of the time) shadorne do a good job of disagreeing without attacking. 

Criticize the idea, not the person..

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.

The cables seem highly unlikely, based on the info provided for a multitude of reasons. It is sudden, it's on both channels and proacman didn't mention messing with cables at all.