I have a VK 212 (not known for reliabilty, I know) and recently have been noticing a low-level, bassy, crackling when i stop playing.

It sounds almost like the roar of a fire (yes my amp is definateley NOT on fire, it was just the best way of comparing it)

I think it started when I once was messing around with the power stage valves (I tried pulling two to make them hotter, didnt work so i put them all in again).
Could the problem be that I didnt replace the valves well enough?