I hear a random low booming noise .

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.

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?

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.

If it only happens while playing music at low volumes my first suspect would be the source. Do you have more than one source to try? Maybe a portable CD player you could hook up to see if it does it too?

Herman, this is a very short and dull "boom" that happens randomly meaning no discernable pattern. I can best describe the sound as like that of a subwoofer, but very short in duration (kind of a pop but at very low frequency). It can be heard between tracks or during, but only in silent or low passages. As far as frequency of occurance goes, it is more of a periodic nuisance than a continuous problem.

Are you playing the 1812 Overture when this happens?

Could you please be more descriptive? Does it boom when playing music or just sitting there? If with music, how loud. What do you mean by random?

Could you describe the boom? I've never heard that term used to describe an audio problem.

Thanks Swampwalker, do you think this could be from a malfunctioning tube in the amp and/or preamp or would this be a symptom of bigger problem with amp and/or preamp?

I think that what you are hearing is the woofer bottoming out from some DC leakage. Not a good thing.