Lately my amp has been acting up a bit - when I dig in around the 3rd-5th fret on the E string, I get a crackling sound out of my cabinets. I've tried different basses and cables with no change. The only thing I've done to my rig lately is swap out the pre-amp tube (I have an SWR SM500) and I'm wondering if somehow I may have just got a bad tube. Or could it be something else altogether? I will try another tube out to see if this helps, but was wondering if there were other ideas out there. Thanks for the help.