Diagnostics for hum

My Celeste HT-3 amp has recently developed a hum in both the left and right channels. What are some basic (non repair shop) diagnostics I can do to help find the problem. I have removed the preamp, and the hum is still there with just the speakers.
Try lifting the A/C ground wire out of the circuit. (use a 'cheater' plug)
Try grounding the chassis of the amp to the A/C ground with a bit of wire..

Try it from a different A/C outlet

My (bad) guess it probably is a dying capacitor. Some small signal level one(s)
Sounds crazy to put a $.99 adapter on a $200.00 power cord, but I'll give it a shot. Do I need to screw the ground on the Cheater plug to the receptacle?
Anything motorized that has been added to the circuit can create problems. Ceiling fans, new gear, moving around interconnects, heaters and air conditioners, computers and so on.

I would first run an extension cord from another circuit to the gear and see if the problem is in the circuit. If the problem still exists, I would check my breaker panel and grounding rod connection to see if anything appears abnormal...ants or moisture, for example.

Cleaning all the connections and unplugging everything and reconnecting it is good audio housekeeping and worth a try.

Think real hard about any recent electronic purchases that you have added to your home that could possibly be on that circuit or polluting your juice. Battery chargers in the garage, for example. Anything with a transformer?
Well, I am rather relieved to say that the cheater plug didn't work. Any other options before the trip to the shop? If I can even find one in OKC.
Sounds like possibly a bad cap in common power supply. Not something playing around with lifting ground, etc. is going to solve.
Thanks for all the input. I guess it's off to the shop, or possibly time to buy a new amp.
Brought it into the shop, $110.00 later it's singing again. Bad capacitors in the pre amp inputs.
Elizabeth for the win!