Why does my system now have shrill top end.

I would seriously look at the amp. High end harshness is a symptom of a class A amp starving for bias. Does the amp run as warm as it usually did?

What the hell is up with you nutjobs and your snake oil fixes????
It sounds like something is wonky with the power supply in the amp. Not cables!!! Not morphic fields! Idiots!

@almarg 
I'm aware that some cables are designed to be highly reactive. I seriously doubt any cable could cause a sudden onset of an undesirable characteristic. A cable may attenuate the problem, but not cure it. 
If I was suddenly having this problem the first thing I'd look for is DC on the outputs. Next I'd verify rail voltage on the power supply and give a good visual inspection of all the discreet components. I'd do the basics to rule out the amp because that's the most likely culprit. 

Awww... Looks like Geoff got butt hurt again for having is snake oil called out. I think I need another tactic. I think I'm just going to report every single snake oil post I come across so threads like this don't get derailed by these clowns.
Happy posting!

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. 

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. 

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. 

@almarg 

I would be genuinely shocked to discover a Pass XA30.8 exhibiting ringing. Nelson believes in wide bandwidth but also takes great care to avoid ringing in his amps. He once built an amp at Threshold that became unstable into cables such as we're describing and it released the magic smoke. Since then he's made it a point to avoid designs with that possibility. The F5 is an exception to that where he specifically warns against exotic cables that are good at transmitting high frequencies, like Litz wire, because of the DC-1MHz bandwidth of the amp. I suppose it's technically possible something could fail to allow that, but I think we'd be talking about a short or a resistor failing closed and that would lead to far more drama than a shrill top end. 

I'm just thinking about how frustrated I'd be getting a lot of advice that doesn't much apply. Been there, done that, not fun. I had a discussion with a mod and I'm making it a point to be less aggressive, but I have no intention of not questioning and challenging things I don't agree with.