What is your impression of vintage Telefunken signal tubes ?

Without getting into some esoteric tubes that may better them, Telefunken are simply the best if you like clear and transparent sound. 

Everyone likes clear and transparent sound but there are other elements of realism that are no less important.

Of course there is no one perfect tube and one type of sound is inherently different from another.  Telefunkens deliver clarity and explosive dynamics but that makes them lean sounding (lacking in warmth).  For the 12 AX7 type, I like the Telefunken ECC 803S which is a bit lean and bright, but it otherwise great sounding.  I run these in my phono stage.  Sadly, they are quite crazy expensive.

Only tube I roll in the Lamm ML2-- the 12ax7. I prefer the ribbed plate only because the smooth does not have the same "grit" and the amps don't need more "smooth"---I have a good stash of these old tubes, NIB/NOS. 

I still consider Joes Tube Lore the best resource for information on tubes:   AudioAsylum.com -- Joe's Tube Lore.  And since we are talking NOS tubes its description is always current.

In my preamp, of the many brands of NOS 12ax7s and 5751s I have used, I prefer Sylvania gold brand black plate triple mica 5751s.  

I agree with Joe; in that to my ears in my preamp 5751s sound better than 12ax7s.  Very subjective