I currently have the stock 12AU7, a Brimar 12BH7, a Tungsram E80CC and a 6SN7 adapter with various 6SN7s (but haven't tried all of them yet). I also have RCA 6AS7G, Bendix 6080WB with solid graphite plates, Tung-Sol 5998, Chatham 2399, Sylvania 7236, Sylvania Gold Brand 6080, Russian 6N5S and a G.E.C. 6AS7G on the way*. And some other 6080 as well (can't remember off-hand).
Part of my problem is that A/B-ing tubes feels almost impossible to me. I listen to the amp with a given set of tubes (after letting the tubes warm up first), then turn of the amp, wait 5-10 minutes for the tubes to cool, then plug in the new tubes, wait for them to warm up, then listen to music again. The differences made by the tubes are subtle enough that it is tough to really get a sense of how they compare when there is 15 minutes in between. So far I seem to like the 5998s best, but I don't really have a preference on the driver tubes yet...
Best,
Adam
* The G.E.C. 6AS7Gs are being sent from the Ukraine. They were supposed to be delivered in 7 days. That was 11 days ago. According to the online tracking system, the tubes are still sitting in Kiev. When I asked why, I was told that the delay was because a major Ukrainian airline (Aerosvit), which was supposed to be the carrier, went bankrupt. Grrrrrrrrrrrrrrrrrr..........................