OK, so I found a nice-looking, nice-sounding Ampeg SVT-IIP preamp for a price I was willing to pay. I really like it so far, it pretty much has the tone I'm looking for, although it'll take a while to really understand it. The tube question is this: This preamp uses 3 12ax7's. The tubes that are in it are probably 10 years old, which is to say original, and they are SLM Hot Rods which are some sort of re-branded Chinese tubes. Would replacing them make a big difference, and if so, what tubes should I use? I'm looking for a big, fat SVT-ish (duh) kind of sound.