Today, I switched to a new-production 12BH7 in the Crack driver socket, and really like what I hear. The tube is much "bigger" sounding, in terms of soundstage and bass, and sounds like a veil has been lifted in comparison to the 12AU7 (actually, 5963) in there earlier. I picked one up from the Tube Store when ordering some other things - $13.95 is a cheap risk - and I ordered the "regular" EH 12BH7, not the fancy gold-pinned one. Who knows, maybe an NOS tube would be even better....
FYI, part of the great sound is probably attributable to other recent changes - for example, the Speedball upgrade and two humongous Dayton 100uF film capacitors instead of the electrolytics. The power triode is a Russian 6N13S (6AS7G). Headphones are Sennheiser HD650, which are finally starting to break in (maybe 100 hours on them?). The caps, CCS boards, and new tube haven't broken in yet, of course, but I can only imagine that things will sound even better with time! The Crack is quite a headphone amp - my new reference setup by far.
