Why does my system now have shrill top end.

Not sure it would be room acoustics as there is no improvement as I walk around the room.

That only works for bass modes. Room acoustics is more than that.

In particular, I’m talking your room’s overall power response. How quickly the top end decays. If it is lingering longer, or reflecting more, it will give the exact issues you describe. So, again, did you move furniture, curtains, etc around? If no, then that’s not likely, but still there is a free experiment you can do.

Throw some blankets or pillows on the floor, especially between and behind the speakers. Experiment. This may not be your issue, but it’s free.

dimmer bad

does it have an all the way OFF detent on it?

dimmers work by 'chopping' up the voltage - this creates a lot of high freq. noise on the line (maybe radiated RFI too)

if setting it all the way down - whether you feel a detent in the knob or not - doesn't solve the problem then either power the lights on a different circuit from the stereo or get rid of them or use an isolation transformer on your system (often a good idea anyway)

if the detent Off doesn't work and you don't need those lights on when listening (i.e. you have another light source), then you can wire in an old-timey (off is off) light switch ahead of the dimmer circuit

Erik’s ideas are all good, but this happened suddenly w/out any room changes...

so do them after finding & fixing the cause of the sudden glare onset

one last thing remove and re- install all your cables as the connections will degrade over time due to corrosion. simply remove them and put them back on looking also for any build up of corrosion and remove if needed.