Why does my system now have shrill top end.

Changing the amp made no difference.

How about swap the preamp?
 

At the risk of repeating myself, this appears to be very much a circuit focused group. It seems my earlier post intimating the problem of the “shrill top end” could be *something else entirely,” something out of the blue, was taken down hard. You know, the suggestion that the culprit could be unrelated to the audio system entirely, I.e., not (rpt not) the amplifier, preamplifier, cables, power cord, room acoustics, or even the house AC. It doesn’t have to be the amp or the preamp. And I am not referring to any of the issues in Vogue such as fuses, wire directionality, polarity, out of phase conditions, things of that nature. But something a little bit scarier. I’ll be watching with some interest to see how this thread plays out. 😀

I am still surprised the OP has not at least tried swapping speaker cables with how cheap, quick and easy it would be to do.......

I added two lights with in-line dimmers to a distant 20 amp outlet that also powers my system. With the lamp dimmers on, it significantly raised a multi-amplifiers operating temperature and added a soft grain in the separately powered mids and tweeters. It disappeared when I turned off the dimmers or unplugged the lights.   

Here is the latest update. I ordered treatment for the side walls and will report back. But wanting to make sure it was not my hearing I visited a nearby audio dealer (Audio Connection) with the same music in hand that sounds so shrill at home. And guess what, it sounds fine in their system. By the way, I listed to the Proac 48r, which I did not like, and the Vandersteen Quatro CT, which I liked so much I may buy them if I can sell my current speakers.