What is your impression of vintage Telefunken signal tubes ?

I’d guess that it’s highly dependent on the application/system, but I have found Tele12AU7s to be very detailed, somewhat clinical…if your system tends toward warm and syrupy then a Telefunken might be perfect. If your system is already leaning toward the clinical side….I’d steer away. 

Nos Tele one of the best worth rolling

My Linear Tube Audio MicroZOTL preamp uses 12AT7 input tubes.  My favorite is the Telefunken ECC801S.  Second place is a Siemens ECC801S.  I've tried several other tubes that sound pretty good, but those are my favorites. Very detailed, but not overly bright or harsh in my system. 

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.