Eliminate system hum

try the granite audio ground zero. it's specifically designed to eliminate hum and reduce your overall noise floor. i've never used one so i can't say how good they really work. call the company and see if they offer any type of money back offer if it doesn't work.

If your Cable TV is run through your system, try disconnecting all cabeling between the two and see what happens.

I found getting an electrician to install a dedicated line, to get my hi-fi isolated from TV and other appliances, was a very cost-effective solution to transformer hum. It cost much less than decent power conditioners, which may or may not provide other benefits.

That said, I also found that speaker cable with the correct impedance for my particular amp (not a tube amp) made a big difference. Installing unsuitable cable, even though it sounded good in other ways, brought back transformer hum.
Removing it eliminated the problem. Your dealer or manufacturer rep may have useful information on impedance matching between cable and amp. Good luck!

-paul-

I have found several brands of EF86 tubes are very sensitive to RF noise. Make sure its not any of the tubes by moving your hand close to them,(do not touch them) if the noise level changes then try another tube.

If you have a Tube Cage, try taking it off or if your not using it putting it on.

You dont say if your using any power conditioning, if you are, try lifting the ground on the amp by using a cheater plug. This might indicate a ground loop.

Try a better power cord on your amp. Perhaps one with better shielding.

I have used the Blue Circle Noise Hound with EXCELLENT results. Just plug it in to any outlet close to the outlets that you use for your tv.