Peter, Have you read "Joe's Tube Lore". I'll give the link below. As you read, remember that he was probably using a different pre amp and gear than you. Also his tastes may be different than your's. So don't exclude a tube solely because he didn't like or list it. With those caveats, I've found his post from the late 90's to be very helpful. Unfortunately, I also think his views have been partly responsible for many of his favored tubes dramatically increasing in cost since he revealed his lists.
I went back and read what he said about Telfunken and Mullard 12AU7 type tubes...I copied it below. This is why I haven't tried one. I have tried a Mullard CV 4003 from Upscale Audio...I liked it, just not better than my favorite Black Plate types made by RCA. It's worth reading his lengthy thread. Mark.
"Now shifting back to the present, the only other comments I might add relate to the obvious "what about the Mullard rib plates & legendary Telefunkens?" question. Well frankly there wasn't much to say. IMHO these are two of the most overrated brands in the NOS universe. generally speaking all the Telefunken 12A family tubes I've heard ('X7s, 'T7s, 'U7s) are a bit bright, thin and airy sounding. I've talked to Charlie Kittleson at Vacuum Tube Valley about this and he basically agreed, observing that these guys more or less built their reputation on their use by the old gear crowd. Why? Well his comment was that the caps, resistors & transformers in old gear were dark sounding sludge and the Teles offered a useful brightening of the sound to a more natural balance. The issue, of course, is that when you put them in a contemporary design with much wider bandwidth and a more neutral '90s kind of balance things can go over the top.
Now before any Tele lovers who find them ideal in their new gear freak out and tell me how wonderful they sound - I believe it. There is a system out there somewhere that's the ideal match for any tube. This is just a comment on a general tendency. Now Mullards are another matter. I have found a few that sound quite nice and they generally are warmer than Teles, but in my rig they never quite make it to the top of the heap."
http://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.html