Looking for everyone's take on what 6550s are "best" and what sonic differences they offer in a Marshall Super Bass. My story: I just acquired a '73 PCB Marshall SB 100 that had fresh EH 6550s in it. Using this primarily for guitar (no surprise there). Amp had incredibly broad range of response on bass control, fair amount of dirt even at volume around 1-2 on either channel or bridged channels. Ran the SB through a Power Soak, dimed amp, choked attenuator to lowest setting ... fizzle, pop, nothing. Replaced 4 amp fuse and took to tech. Tech said one of EH tubes blew and made remark about them being Russian crap. Without really consulting with me as to the sound I was going for, he decided to go with Ruby Tubes because he likes them and has good luck with them. Now, the amp sounds quite a bit cleaner, I feel, and the bass control does not feel nearly as responsive as "rangy" or "big" as it had. I've since found a couple of comparisons of 6550s: 6550 Tube Selection Guide - Tubes for Amps www.thetubestore.com - 6550 Tube Types Your thoughts? Thx!