Last month I bought a used Ampeg Superjet RI (model SJ-12R). It had a little exterior wear, tarnished control plate, but heck it was under $200, so I figured I'd give it a go. When I demo'd it the tone was thin and the tone controls had little effect on shaping the sound. Figured "probably beat tubes, and I'll probably swap speakers anyways.." Now, it had EL-34's (Ampeg eventually went to 6L6's in these). Well, I prefer 6l6's, so I figured I'd swap them with all fresh tubes. Called Eurotubes....they said no problem, but I should bias them. New tubes come in, I set the bias, all appears good. After a few minutes of playing....pops, static, occasional hissing. Present even if there's nothing plugged in. Again, tone controls did very little, if anything to alter the tone. Bass and treble? Fuggitaboutit! Dropped it off at the tech. It's a reliable shop, plus their an authorzed Ampeg repair facility. Two weeks later....tech calls..."well, the plate voltage on these really isn't meant for 6L6's". He swears he tried numerous sets of EL-34's and couldn't make the amp make any unusual noises. Only with the 6L6's was noise an issue. For good measure he re-traced the solders on the boards and replaced both preamp tube sockets. In;ut sockets checked ok with the exception of one bad solder, which was touched up. $75 later............ Took it home, popped in the old 34's...mucho popping, static. Tried the 6L6's...ditto. Now, the preamp tubes were also new JJ's. Tried at least 6 different 12AX7's I have laying around (that all have been fine in other amps)...some more static than others (?). Only a couple of the preamp tubes allowed for any adjustment in the eq, with the exception of the mid control which goes from cut to HONK. I cleaned all of the pots, even swapped 2 different speakers...nada....same ol' scratchy, static filled noise. Oddly, with one of the speakers the noise was much more pronounced if that matters.. Now I'm at the WTF stage. What's the source of the noise??? Lastly, what's up with the tone stack??? I cannot believe the 2 bands of eq simply have knobs for decoration. There's gotta be something somebody is missing here. Granted, I'm no electronics tech by any means, but I suspect there's another possible cause. Any clues? I'd really love some advice..