Why does my system now have shrill top end.

 LED bulbs is likely

turn them off and see if that fixes things - if not, post exactly what type they are (built in or replacement screw in type)

Also if the LED lights in the kitchen are on a dimmer switch that's a big culprit of AC line noise.

They all are on a dimmer.

I don’t know what the capacitance per unit length of OCOS speaker cable is, but given that your cable is fairly long and given also that OCOS is a somewhat unconventional design a possibility that occurs to me is that its capacitance might be causing an ultrasonic oscillation in your amplifier. And perhaps the condition is sufficiently marginal that minor aging effects or perhaps even a change in AC line voltage have put it over the edge.

If so, the volume dependency you mentioned might result from the speakers being stressed to a greater degree when an oscillation and a high volume audio signal are both present.

So I would suggest trying different speaker cables, whenever that may become practicable.

Good luck. Regards,
-- Al

Oooh, dimmers can definitely make trouble.
I recall other threads on this subject.
Since this occurred after the kitchen upgrade, I have a sneaking suspicion that the electrical work has something to do with the sudden change in your stereo.
Perhaps Almarg will chime in....
B