Alright I've had a chance to toy around with some tube rolling and thought I'd post some results for those of you that might be interested. I guess for the sake of clarity I should mention my setup which is APL Hifi DV-563a > PrimaLuna ProLouge One > Magnepan MMGs, using magwire interconnects and Chris Ven Haus recipe DIY speaker cables. I should also mention I used Svetlana C logo EL34s for the entire comparison. OK here goes:
12AU7:
RFT 12AU7: These were purchased from Ned at Triode Electronics for $14 a piece, I think I snagged the last matched pair but there are a couple more singles available. These are the newest of the lot dating from the 1980s and have a very different internal structure than the rest of the tubes (smaller plates, more simplified design.) My initial impression about these were that they were pretty well balanced from top to bottom with nice detail throughout. They had a pretty well defined soundstage with nice separation, more so than the rest of the lot. Also these are pretty transparent without a whole lot of warmth, I really liked them with the Sylvania 5751s but not quite as much with the Tele 12AX7s. If I could only own one type of 12AU7 this would probably be it, while it isn't the best it is the most consistant.
Cifte 12AU7WA: I picked these up from Upscale Audio for $25 a piece after talking with Kevin Deal. I believe they were made in France in the late 1950s or early 1960s by Mazda. My initial thought about these were that they seemed just a little thin and touch bright which is actually what I was looking for. These were a perfect match for the Sylvanias because the they didn't over emphasize the midrange like some of the other tubes and added a nice high end sparkle which I felt was missing from all of the others. On the other hand these didn't mate well with the Telefunkens, the sound was way too bright and thin. I think these are very system dependant and in most circumstances probably wouldn't be the best option. As always YMMV
RCA 12AU7 Cleartop: I found these dirt cheap on Audiogon ($5 each) so I decided to pick up a quad to see if they were really as good as advertised. I was immediately impressed, they have a very nice smooth range with a beautiful midrange, above average highs and very tight bass. The soundstage was pretty good, they're definately more musical than the RFTs or Ciftes and probably on the same level as the Amperexes. I really loved these with the Telefunkens, I think they're pretty close to the perfect combination of detail and musicality. Once you factor in the cost these are a no brainer, I highly recommend them.
Amperex 12AU7A Orange Globe: These are the late 1960s - early 1970s variety which followed the famed Bugleboys. These were by far the most expensive of the lot at $40 a piece which is actually pretty low for these. The first thing I noticed was the very smooth and musical midrange much like the RCAs but these had a little more extension on the highs. All considered these sound just slightly more detailed than the RCAs without sacrificing the natural, musical sound. These were probably the overall "best" in my system though I'm not sure they're worth the dramatic difference in price.
Overall my favorite combinations would probably go like this:
1. Telefunken 12AX7 > Amperex 12AU7
2. Telefunken 12AX7 > RCA 12AU7
2. Sylvania 5751 > Cifte 12AU7WA
The differerence between the Amperex and RCA 12AU7s was fairly minor, with the slight edge going to the Amperexes. While it isn't in any of my "favorite" combinations if I had to choose only one 12AU7 it would probably be the RFT because it sounded pretty good with both the Sylvanias and the Telefunkens, unlike the others. Unless you really need to brighten up your system I'd probably avoid the Cifte because I think in most circumstances it would sound way to bright and would be downright fatiguing to listen to. I think that's all I have to say on these, hopefully it was somewhat helpful to those who are looking to roll 12AU7s. I'm currently looking to add a couple 12AX7s / 5751s and I'll share some thoughts on those after they arrive.
