Why does my system now have shrill top end.

I have not read every post in this thread, but how old are you, and have you ruled out hearing loss? - Gerry

Al,

I agree the speaker cables should be replaced with something else to conduct a test. The most the OP will be out is a few bucks and a little of his time. The OP can pick up something cheap speaker cable at Best Buy or Home Depot.

Al,

What are the chances the OP could connect the Linn Linto Phono Preamp directly to the inputs of the Pass amp without it playing too loud for a test. That would take the Pass XP 20 out of the equation.

SPECS

Input connectors: gold-plated direct circuit board mounted RCA jacks
Output connectors: two pairs jacks (as above)
Audio input level: 150 microvolts nominal, High Gain
500 , Low Gain
Input Impedance: 150 Ohms, in parallel with 4.7nF
Output impedance: 100 ohms both pairs of outputs
Gain: 64dB at 1KHz, High Gain
54dB at 1 KHz, Low Gain
Power consumption: 6 watts, 12VA approximately
Dimensions: 320 x 320 x 80mm
Weight 1.5kg

https://www.audaud.com/audaud/JUL-AUG02/EQUIP/equip2JUL02.html

Unplug the Pass XP 20 from the power outlet.

 Try the Linn Linto Phono Preamp first plugged into the P5. If the " shill" sound is still present plug the phono preamp directly into the wall outlet. Unplug the P5 from the wall outlet.

Jim

How old is your house?
what kind of AC outlets occupy the house?
How many GFCI outlets in the house?

How many GFCI outlets in the house?

If a GFCI can cause a problem with the sound from an audio system, audio equipment designer/manufactures best be solving their problem why.

All I can say is what worked for me...
dont want my advice, don't take it.