Fender Champion 600, ground hum.

[minnehond]

I bought a Champion 600 last weekend, it sounds great, especially after I put some better tubes in, but there's one problem.

 

There seems to be a grounding issue. It hums and hisses, increasingly so when I turn op the volume. Whenever I touch a metal part of the amp (such as the back panel or the switch) or the strings or bridge on my guitar the hum is reduced to somewhat more bearable levels but it is still there.

 

Now, my guitars are properly grounded, no problems with any of my other amps. Cables are fine too. Moved the amp to another room, no change. So I'm thinking somethings wrong in within the amp. Weirdest thing is that this problem is more apparent with my humbucker equipped guitars than with my Strat.

 

I don't mind opening up the amp but I would like to know beforehand what I'm supposed to be looking for. Or maybe this is a problem someone else here experienced. Has anybody got any idea 'bout what the problem might be and how to solve it? I read something about "lifting the filaments" but I can't find proper instructions on how to actually achieve this.