|If you're going to spend $5k-$6k, get the best. Don't settle for 12AU7 and 12AX7 tubes.
But what is the best? One of the biggest problems with audio is my best is different from your best, which may be different from a thousand other bests I would like better than my current best but which I've never heard and may never have a chance to hear.
For example, if my MPX wasn't an SE version, where I can use 6bl7xgta tubes, I would have been very disappointed with the amp. The 6SN7 and SLAM-based output tubes (the same ones the B-52 uses, I can't remember the #) are boring by comparison. Same with the inputs. If I couldn't use the Tung-Sol 2c51 on the MPX and Extreme (and certain 12AT7's on Extreme), I would be also be missing a great deal of the capability of these amps.
That's why I would never buy a B-52 or this Millett 307A now without being able to directly compare to my current setup beforehand. I don't like the B-52's output tubes on my MPX at all. And I don't like the input tubes of the 307A on my MPX or Extreme compared to many others. Can the rest of the amp make up for the dislike of the tube choices? I don't know and would not take the risk blind given my current setup. That's one of the things I really like about SinglePower amps, is that I can easily try over six major classes of input tubes (and many more sub-classes), and several types of output tube classes to figure what I like best.
I'm still trying new things, but my current setup is the best I've heard so far. And while I'd love to go to some of the meets and try these different things, I know from experience that it usually takes me days and weeks to truely figure out if I like a certain component or not. And it's usually next to impossible to effectively evaluate something without direct comparison to my reference. I just don't have that good of sonic memory.