I have a Mesa M2000 that I just got from the local GC (nos) for a price I couldn't pass up. I've had it for approx. 2 weeks & I'm starting to notice a low hum when the amp clicks in and what sounds like intermittent, background low-pitch "crackle". The "crackle" is not very loud, but it's there. The amp is working, with no drop in volume, but I was wondering if it could be that the tube that's in front of the mosfet could be going bad? I get the "hum" and "low crackle" on both channels. I get it probably 80% of the time. Once in a while when I turn it on, it's perfectly quiet. Any ideas? Is it probably the mosfet tube? Thanks!