Why does my system now have shrill top end.

A $20 DMM is all you need to check the DC offset, bias, and rail voltages. If you have a problem you can hear you're talking variations of at least 75mV. If there is DC at the outputs, try the amp at a friend's house to rule out the amp. If it still reads significant DC, it's the amp. If it did show DC and stops elsewhere, then there may be something drawing significant current off the mains using half-wave rectification. That would put DC on the AC mains. Half-wave rectification is common with LED lighting and small, cheap power supplies. I'd also look for DC on the pre-amp outputs. Even a few mV would be a problem. 
That's the kind of troubleshooting I'd be doing. Everything except checking the bias and rails is cheap and easy and rules out problems. I think anytime you have a stereo change it's sound you should look for DC on the outputs. I wouldn't assume an amp like the XA30.8 would have rigorous DC protection due to it's very low bandwidth limit. 

@almarg 
The amp in question is PP. It would be unlikely for the bias to drift and not create some offset. 

Fascinating how the circuit focused group analyzes the problem as an amplifier issue and dismisses all other possibilities pretty much out of hand, and rather forcefully. A microcosm of how audiophiles tend to think?

Kosst, thanks for the clarification about the multimeter measurements.

I also want to thank Jwpstayman for his very informative post about the OCOS cables. For one thing, it further reinforces suspicion that they have ultra-high capacitance. (I believe that when he referred to a "nobel network" he probably meant to say "Zobel network," which is something that is commonly used with ultra-high capacitance cables such as Goertz to minimize the capacitive nature of the cables as seen by the amp, that might otherwise cause ringing, oscillations, or even damage).

So based on his post a question for the OP would be if his OCOS cables have a "red" termination network, a "black" termination network, or neither.

Regards,
-- Al

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