It is senseless to talk about good and bad vacuum tubes without knowing the "house" sound of each brand which can vary according to production periods. If a tube seller doesnt ask specific details about your components as well question you regarding the desired sonic results they are doing you a disservice. Find someone that knows tubes as well as components and is more interested in finding out about you as a hobbyist than selling you what he has in stock.
These forums are a great source of information, but avoid comments like you are fine with any such and such a 12AX7 and specify JAN tubes as being sonically different than civilian made tubes. Usually JAN tubes are the exact same internally as are the civilian versions. I think tubes made for medical devices or organs and perhaps other equipment have a better chance of sounding better than tubes for the military. Many comment on the quality of NOS or new production tubes with very little tube rolling experience or are rolling in equipment where minute changes cant be heard. Its interesting that some attest to the different sound of tubes but are also doubting or denying the affects of cables or other tweeks.
Every tube seller should tell you is that different tubes in different sections of just about every application do not affect the sound of the component equally even why they are of the same type and from the same mfgr. Whenever retubing a customer's power amp I usually start with the front end and then deal with the power tubes. My experience is that these driver type tubes always affected my tube amps more than changing the big guys.
FWIW: the 2 best more common types of old stock 12AX7s that I like are the Telefunken smooth plate and Holland made Philips Europe 12AX7s. Sometimes I will sell pairs of these to the same client for the same component. Mixing and matching 6SN7 pairs in the Atma-sphere amps is a great way to combine different sounding tubes in the same circuit.
