Alright this thread is for all those interested in sharing or learning about the tonal possibilities related to swapping different tubes through the Blue Tube 903 pedal. Blue tube rack mounts and others are welcome to chime in. Just as background, I'm not going for an actual overdrive fuzz tone. I usually play R & B, funk, 70s & Motown, and current top 40 cover tunes, so I go for a clean punchy tone using my GK / Eden / EA rigs. Once in awhile, I jam with some old school 1960s style classic rockers, and I find that my rigs are too clean and punchy. So, rather than buy an SVT, I thought I'd try a tube preamp / overdrive pedal. I seem to be getting close to the tone that I'm looking for, just by dialing in a little overdrive for warmth and grit. My 903 pedal came with a cracked Ruby 12AX7, which I never got to hear for obvious reasons. A friend immediately got me a used Mullard 12AX7. This sounded OK, it seemed to have a bump in the lower mid-bass, slight harsh overdrive when the drive nob is tunred up. I think this tube may have been better suited for lead guitar. Next I bought a Groove Tube 12AU7. Slightly less gain, warmer than the Mullard for bass, nice understated overdrive. I try to get some samples up in this thread this weekend (already have one up in another thread). I'm also interested in trying a 5751. I've heard from some bassists that claim this one has nice warm lows. This weekend a bassist/guitarist tube guru-type is gonna provide me with a 12AY7. He claims it might get me closer to the B-15, Bassman, kind of tone (yeah, yeah, I know I should go buy one of those heads). Anyway, we'll see how it goes.