Probably electrolytic capacitors in the power supply.
Look for bulging or leaking large electrolytics.
Plenty of info about what to look for on the internet, possibly even a video or two.
Perhaps but are you using say, another preamp or coupled together some gear of your own..??
Independant power supplies, what are you feeing into it? Line or signal levels?
So far all we know is, it's a box with an output that is distorting.
Never heard so much old wifes tale bs in all my born days.
Current generation has much to be thankful for now all you home grown "experts" are out of the main loop.