Why does my system now have shrill top end.

Excessive DC on the amplifiers output can easily be checked even without a multi meter, just turn on amplifier while observing the woofer cone if it moves out and does not return to neutral position after the initial turn on bump - the amplifier have excessive DC on the output. It of course can move in also in case the DC voltage is negative.

Opening up the amplifier and checking rail voltages and Bias current is on a whole other level, and best left to someone whom knows what they are doing.

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 of Luck
Peter  

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.

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