FWIW, I've toyed around a little bit with different tube combinations. With my Audio Experience Symphonies > McCormack DNA-1 > B&W DM610i and now Von Schweikert VR-1s, I've tried:
Input: Electro Harmonix 12AX7; Output: JJ / Tesla 12AX7
Robust bass, but a little sloppy for my tastes. Mids a little muddied, and highs a little spitty and fatiguing. Didn't like this combo at all.
Input: GE JG-5751; Output: GE JG-5751
Came across pretty thin in the bass, sweet and detailed midrange, cleaner and clearer highs than the 12AX7 tubes. Overall, too thin anemic for my tastes, though I loved the midrange detail.
Input: JJ / Tesla 12AX7; Output: GE JG-5751
Helped fill out the bass a bit more, less midrange detail than above, still pretty clean highs. I liked this combo a fair amount. Until...
Input: Ei Elite Gold; Output: GE JG-5751
My favorite combo so far. Much fuller, more robust bass - woof. Retains nice midrange detail, and the highs are a little more recessed than previously. Highs maybe a little too recessed, but I have some listening to do before I swap around again.
So far, so good. This is fun!