What is your impression of vintage Telefunken signal tubes ?

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.

It almost sounds like they are kind of 'military' tubes, nothing to do with art. No one mentioned great midrange and soundstage. 

I’ve used the vintage ECC83’s in all my tube amps over the years and they are by far the best I’ve ever used. Fantastic detail and decay, great soundstage, etc. 

They're my go-to when possible. 

telefunken small signal tubes (real ones from back in the day, not the modern branded tele’s) are excellent, though they are in the clear and precise school of sound with excellent air and imaging, not very overty tubey in their sonic nature

to me they are not better than other top european tubes from that era, mazda’s adzam’s siemens cifte’s dutch amperex and even hungarian tungsrams

somehow the tele’s became ’legendary’ and demand for them went off the charts... made them poor values to buy, even in the 80’s and 90’s