What is your impression of vintage Telefunken signal tubes ?

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 

Yeah I now use RCA/JAN 5814A in a tube DAC vs the "better" 12au7 tubes I'd been using. They seem cleaner and give instruments & voices within the soundstage more definition. If that makes sense. I loved Telefunken smooth long plate 12ax7s in my Calypso Signature but have gone to Mazda chrome-plates now and yikes! In my opinion, nothing compares in that particular preamp - can't believe how much I paid for these two tubes but, totally justified by the positive change in my setup. Just my opinion

I still have a bunch left over, including a few of the ECC802S and 803S.  The best tubes I used in my Jadis JP80, JA80s and Lamm ML 1.1s.  Very clear, dynamic and long-lasting in those applications.  I would also say I think they have a more neutral tonal balance than some of the others I tried, but again it may just be the applications.

I just purchased a pair of 'expensive' 12AX7 tubes for my Zesto preamp. 

During my phone call with the seller, who is very well known and respected, he advised me to not purchase Telefunken because they would sound too dry in my system.

This is not personal experience, but everyone here trusts his advice.  YMMV.