does it only buzz when its in the dirty channel?

is the texture in class A or A/B?

how loud is the amp?

its not your preamp tubes, more than likely. it'll take years for them to screw up.
computers will mess with your guitar and make a sort of buzzing sound, but only if you're pretty close to it. also, i've had my wireless house phone on top of my amp and have it ring multiple times, without problems...

try moving your amp around the room to find a nice spot. not all places in your room are created equal.