Why does my system now have shrill top end.

Jim (Jea48), in my case there's no particular reason I haven't mentioned the preamp.  It's certainly one of many possibilities that can't be ruled out at this point.

Also, +1 to all of the comments by Uberwaltz.

Best regards,
-- Al
 

The cables seem highly unlikely, based on the info provided for a multitude of reasons. It is sudden, it's on both channels and proacman didn't mention messing with cables at all.

@toddverrone

I would agree the cables or the interconnects are unlikely culprits but as it is a very simple and cheap test it should not be ignored.It is possible it really was not sudden, a connection may have been corroding or degrading for a while but then reached a point where it had gone that far as to now present itself as an audible artifact at last.Unlikely true but not impossible and one that should not be ignored in the greater scheme of things.

I completely agree with Todd. I find myself befuddled by the persistent recommendations that the least likely culprit, the speaker cables, be given first attention. It's free and harmless to connect a DMM to the amp outputs and determine if the bias is off. 

I also agree, Todd, that the cables are not a particularly likely culprit. However, as Uberwaltz said just above, and as I said earlier, it is easy enough to perform a test that would rule the possibility in or out. And as I also said earlier there are very strong reasons to suspect that the OCOS cables have very high capacitance, and ...

Almarg 11-27-2017
...a possibility [note that I said "possibility," not "probability"] that occurs to me is that its [the cable’s] capacitance might be causing an ultrasonic oscillation in your [the OP’s] 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 [the OP] 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 to be sure it’s clear, the **possibility** I suggested was not that the cables might be the culprit in themselves, but rather that the effects of their presumably very high capacitance on the amplifier **might** be. And, once again, it’s a possibility that is very easy to check.

Also, a reason that this possibility has been given what Kosst characterizes as "first attention" is that we certainly want to be sure that it has been conclusively ruled out before other findings may lead the OP to incur the trouble, expense, and risk of sending his amp out for a repair that may prove to be unnecessary.

So while Kosst finds himself "befuddled by the persistent recommendations that the least likely culprit, the speaker cables, be given first attention," I find myself befuddled by the objections that have been raised against that attention.

Regards,
-- Al