I have a similar problem:
I own a Marshall TSL100, and it sounds amazing (tone-wise, but there are other issues). It (like those posted before me) recently started a little hum/buzz, and like those before it is much more audible on the clean channel. I know it is not the outlet or any cable, because I've tried several outlets (even at different houses) and also the hum occurs even when there is no cable plugged in the input.
I am going to try tapping the tubes as was suggested, but I was wondering if there was anything else that could be wrong, like my input. Sometimes I notice I have to jiggle the cable plugged into the input to get sound, and I don't think every cable I own is just crapping out, I think it that might actually be my input (although none of my cables (planet waves) have been purchased in the last year/year and a half).
So I'll check the tubes, but if I don't discover anything could my input be the source of the problem with the hum???