Yes, as Swampwalker said, or rather, let me paraphrase; Hum Happens. Not a defect or necessarily worn out (Could be loose bolts, which you could address). Could also be a drop in voltage (I think when you get on 10% or so you could induce some hum). Try the solutions I offered and see if that works. I guess that does deal with the symptom, but if it's just a loose bolt you've dealt with the problem. Try the weights if the bolt doesn't work. Could be tubes (remote chance) too. I'd look at the trannies first - not as a defect, but just for something loose or vibrating. I'd strongly suspect that given that it stops when you touch or move them.
I know just enough about this stuff to sound like I know what I'm talking about, or rather, just enough to be dangerous. To try to address your question, and someone will have to correct me on this if I'm inaccurate, but I think a tranny could break down (some of the binding element they use inside) and that could have that effect of exacerbating hum, BUT, as I understand it, that would be over a greater period of time than an Almarro amp using 6C33C tubes could be old (in other words, not likely). Still could be vibrating. I have a friend who owned those amps for a while. I'll ask him and point him to this thread.
I know just enough about this stuff to sound like I know what I'm talking about, or rather, just enough to be dangerous. To try to address your question, and someone will have to correct me on this if I'm inaccurate, but I think a tranny could break down (some of the binding element they use inside) and that could have that effect of exacerbating hum, BUT, as I understand it, that would be over a greater period of time than an Almarro amp using 6C33C tubes could be old (in other words, not likely). Still could be vibrating. I have a friend who owned those amps for a while. I'll ask him and point him to this thread.