I'd first be looking at the load you are driving.
I assume the use of 2 speakers,, maybe check the total load is not below 4 Ohm.
If 2 speakers are used try running one speaker and see if it distorts.
An 8 Ohm load (1 speaker the power devices will not run as hot and far less chance of distortion due to running hot.
One other possible cause is the bias is drifting when hot leading to Xover distortion.
i don't know the XR circuit but it may have a small trim pot inside for adjustment and as the unit ages that part might be effected by heat.
Talk to the teck guy again and ask him to check,, it may just need a clean of the trim pot.
Phil.
I assume the use of 2 speakers,, maybe check the total load is not below 4 Ohm.
If 2 speakers are used try running one speaker and see if it distorts.
An 8 Ohm load (1 speaker the power devices will not run as hot and far less chance of distortion due to running hot.
One other possible cause is the bias is drifting when hot leading to Xover distortion.
i don't know the XR circuit but it may have a small trim pot inside for adjustment and as the unit ages that part might be effected by heat.
Talk to the teck guy again and ask him to check,, it may just need a clean of the trim pot.
Phil.