Enzo... not sure if you are still out there, but if you are, I just want to say THANK YOU!  I was having the exact same problem as the OP, loud buzz when power applied, regardless of the volume level. 

I've never worked on an amp before, but am electronically inclined. 

So long story short, after about 30 minutes of taking off the board from the chassis to get to the solder joints, I found one of the two big cap's solder joints was clearly visibly "cold" and loose from the solder trace.

I gave all four joints (two per cap) a re-solder and tested it out... No more buzz!  Just clean sound again.

So, once again, from the bottom of my heart... THANK YOU!!