Why does my system now have shrill top end.

+1 Jim (Jea48). Although it may seem improbable, I would not by any means rule out the possibility that BOTH amps have succumbed to the very high capacitance of the speaker cables.

Also, I would add the following to the quotes you provided, this one coming from a respected designer of highly regarded audio electronics and speakers:

Pbnaudio 12-1-2017
My gut reaction to this is the very long, highly capacitive speaker wires are at least contributing to this. As someone stated above - try lamp cord.

Best regards,
-- Al

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.