Mechanical buzz coming from the chassis

I have the amps sitting on 1/4" thick cork mats. This helps, but regardless the hum is not audible from my listening position.

The other strange thing is that only one amp hums but both amps had relatively loose screws.

Also, soft mats and tightnening screws will eliminate the symptoms but do not address the root cause.

Nick, the root cause is that all transformers hum, to varying degrees. And differing manufacturers deal with this differently, some use compliant mounting and some just torquing the tranny down on the chassis. Some trannies hum more than others. It's normal and probably more an annoyance than an actual problem.

Just for kicks and giggles, try using a cheater plug on your power cord and try it both ways in the outlet.

Double check your bias also.A tube starting to short can cause this too.If this amp is easy to bias,you can try swapping the tubes between the two amps.If its hard,it may cause a problem that's not there.

What Viridian said. They generally all hum, but some more than others. My tube amp is totally silent, but it is the exception.

It appears that the hum is due to the bottom plate on the chassis casing vibrating against the side wall. I placed something underneatht the amp to pressure on the bottom cover and this has eliminating the hum.

I guess I could also loosen the screws and place a bit of blue tack between the two surfaces and then tighten everything again.