I hear a random low booming noise .

If there is DC leakage, that can be a problem. I had same issue once with a pre. It was a bad resistor; shipping cost more than the repair. Try to isolate it to one component, then you will probably have to bring that unit to repair.

Does it happen in just one channel of both at the same time. Herman raises an interesting point about the source.
Have you found any additional information to share?

the dc leakage sounds like the problem have you tried moving the valves around that would isolate a valve issue mine had random click which disapeared when i move the left outer valve to the left inner valve position (the front ones) if you tap the front outer valves gently you will here it in the speakers that is how sensitive they are. isolate the amp much the same way as you would a turntable, valves are very suscetible to vibration mine sits on two hefty car washing sponges works a treat.