I've been having some problems with my recently aquired SD18. I've been getting noise, loss of volume/gain, and just sounding dull. Sometimes its great, and other times not. I bought it off Ebay a couple weeks back, and it's been doing it since. I'm not even sure what its really supposed to sound like. I emailed Jeff Bober and he said it sounded like a dirty FX loop so cleaned all Jacks with contact cleaner, and its still the same. I started to switch out some Preamp tubes, and I noticed when I took of the tube cylinders the sound would drasticly change. Now it didn't do this everytime I took off/ put on a cylinder, but after taking them off and replacing them about a dozen times, the issue seems to be pretty random amongst the tubes. It didn't happen with the same tube every time. Anyone have any suggestions.