What are some good 12AX7s to put into my recently acquired used SWR SM-400s? It's currently got a Groove Tubes in it, and while I don't know how old it is, I have noticed something distorts for a few minutes every once in a while and then goes away. I'd like to replace the tube to see if this helps at all. So what are some different characteristics of different types of 12AX7s? (I'm not really sure what I'm looking for because I don't know what tubes do to the sound, I'd like to hear some descriptions first and then pick) Do preamp tubes really have anything to do with the sound at all, or should I just try to find the cleanest one? What about a good place to order them from?