I just spoke with Brent Jesse (audiotubes.com) regarding NOS 6SN7 tubes. He has an impressive supply of very high quality NOS tubes of all types. He allows you to buy, try, and return for upgrade or refund (-20% restocking fee). Although I didn't end up buying from him this time (I found what I wanted locally), I plan on doing so in the future.
How do you test different tubes?
I hear people recommending different brand of tubes for amps, preamps, etc. My question is how are people doing this? Are there dealers who loan tubes like other gear? I would hate to have to purchase different sets of tubes, try them, find the one I like best, and then trying to sell the set I don't like.