Slight hum from SLI-80 power amp

Swapped the tubes. The very slight hum stayed on the right channel.
I replaced the rectifier tubes with 5AR4's [had previously 5U4's] and readjusted the bias to 80 mA. No change. I did install in parallel a REL Acoustic sub-woofer system as per wiring instructions over ONE year ago with no problems. I wonder if there might be a grounding issue somewhere.  Very frustrating. I am totally baffled.

You already swapped the speaker cables,right?Then swapped the interconnects?Take the sub/subs out of the system and see what happens.Take away the source leaving just pre and amp.Next just try the amp and speakers together with no pre or source.You're eliminating one thing at a time to hunt down the hum.Be sure to power down everything each time before pulling out cables!!!

Try cheater plugs on each piece in your system, starting with your amp.

I'm no expert but I had the same type of problem with an amp and on the advice of amp builder Kara Chaffee I tried the cheater plug solution and solved the hum problem.

In her words, "One thing to do is to run the entire system from one outlet in the wall no matter what . The other is to use as many cheaters as needed. You will also have better sound if you can get rid of the loops.  Ground loops are a case of multiple components fighting over who will be the real ground."

JTCF: Have not swapped the speaker cables. This sound system has been working like this for over 1 year.  Why now this?  Your suggestion to remove all external connections like the sub-woofer and CD is a great idea. Your last sentence is very important. Thanks. I will try......

CDC2: I was told not to use a "cheater" to eliminate a possible ground loop,  but your suggestion (or should I say Kara Chaffee) sounds logical.
The sub-woofer is on another outlet since installation days over 1 year ago, but ......  I will follow your suggestion and advise......

Some people are able to solve a ground loop problem by lifting all of the grounds with cheater plugs except for one.IN THEORY everything is hooked together with only a single ground and you should be safe from being electrocuted in case of a short...makes me nervous.Get a professional opinion first.Please!