What makes the telefuken tube so special?

Hi This question will probably have mixed reactions. I always see the teles up for alot more money than other NOS tubes. Since I have not been able to get my hands on one what makes this tube so special? Thanks
The sound is rich, full and musical. Tube audiophiles have been using this tube as a reference for years. They are expensive because they are hard to find.

I have 4 12AX7's that I can't use because I've changed gear and my new preamp uses 12AT7's. Contact me if you're interested.
ah tube sound I have heard that one reason NOS tubes sound better is that the chemicals used long ago are very toxic and can not be used today. thes are the things they use to dope the electrodes . I do not know if it is true but seems a good reason could be true ? lake 513
Richness, Soundstage, Bass extension.
Add longevity too, at least for the earlier ones. To my ears, the Teles are a sonically balanced tube, not favoring any frequency range over another. They sound glorious in Jadis equipment, much as Rayhall and Xon57 described. I have used them and Mullards in my Audio Logic DAC as well, and the Mullards are just too bright in the upper midrange for my tastes--maybe a little more "exciting" because of the emphasis on the upper frequencies, but not as rich and well-rounded as the Teles. Xon57, what vintage are your Teles, and how much use have they gone through? I might be interested if South 43 passes on them.
I have also been told that the tolerences specified on the materials used to build the tubes, and the chemical purities of the componds used were MUCH greater than then today.