Why does my system now have shrill top end.

There is a possibility of a blown fuse inside the amp?
My BAT has 2 fuses per rail and has an annoying propensity for blowing the front fuse on the lh channel rail.
When this happens I can measure over 5vdc at the output terminals. No I do not know why it does this, neither does bat.
Could be a possibility that would preclude and expensive trip to a dealer.

Maybe the tweeters are blown or ferrofluid dried out. The ring radiator is crossed over at 1.8KHz so it works very hard.

My problem with the cables attention is cables don't just magically change character in a hurry. We're looking for something that can change over a short period of time and effects both channels. The power supply of the amp is the most likely culprit. It's best to rule out the most likely first. 

Kosst, my hypothesis is not predicated on the cables having changed in any way whatsoever. If it wasn’t clear, when I referred to the condition being "sufficiently marginal that minor aging effects or perhaps even a change in AC line voltage have put it over the edge," resulting in ringing or oscillation, I was referring to minor aging effects in the amplifier. Which in turn might have made the amplifier more prone to ringing or oscillation as a result of the heavy capacitive load.

Admittedly, though, the fact that both channels are exhibiting the problem does **somewhat** lessen the likelihood of my hypothesis accounting for the problem.

Also, Peter (Pbnaudio), thanks for your comment. As many here are aware, Peter is a highly experienced designer of well regarded audio electronics and speakers.

Regards,
-- Al

Have your listening habits changed? My system sounds much brighter if I listen when I’m fatigued/tired.

Otherwise, it’s most likely that your amp needs service, or you need to listen with a Shakti stone taped to your head.