Is the noise about equal in both channels? If so, it is unlikely to be a bad tube or some other candidate, like a power supply capacitor.
If it is just one amp, or predominantly one amp, try switching the tubes from one amp to the other to see if the noise follows the tubes.
Even with power supply conditioning, it could still be the power. I get hum and buzzing in my amps (Audionote Kageki) from the cable tv feed. As long as the cable is hooked up, it somehow feeds noise into the circuit that feeds both my stereo and video setup (the setups do not share any components/connection at all). Even attempts to block the noise with a blocking transformer doesn't help; I have to unplug the cable to eliminate the noise. The amps are fed by a Furman balanced power conditioner, but, it does nothing for the problem.