OK, I got a used SM400s less than 2 weeks ago, and it has already cut out on me several times. The sound will just completely drop out randomly. To get it back on the last time I cranked the preamp gain all the way up and turned the master volume up pretty high. When I tapped on my instrument cable (not plugged into my bass) it did make a clicking sound, so then I plugged my bass in. BOOM, after nearly blowing out my speakers, the sound came back. Could this have something to do with the preamp tube? I noticed a similar popping kind of sound when I'd remove it with the amp on. Hopefully it's a simple matter of just needing a new tube. Any input would be greatly appreciated.