I have a Cary SLA-70 in which I have tried EL34s, 6550's and KT88's. By far the best sound in my system has been with the Gold Lion re-issue KT88's. Very robust, quiet tube, with just the right touch of 'sweetness' in this Class A circuit. I know there are obviously variations among these various tube types depending on what actual brand of each you try. My EL34's are Manley-branded Groove Tubes (Sovtek, I think - nice, but a little "soft" sounding in this amp), the 6550's are SED 'winged C' Svetlana 6550's (nice, with more extension at both extremes, but somewhat "dry" sounding) and the aforementioned GL KT88's (a great combination of the strengths of the other two). Mine came from Jim McShane, who has acknowledged some quality issues in very early versions, but stands by their reliability. They're very cool-looking to boot :-)
As far as 12AX7's go, you'll find as you research things that the Telefunkens are supposed to be some sort of 'Holy Grail' 12AX7, but I personally find that, though reportedly a great match with thicker, slower, darker-sounding vintage amps, they take away some of that tube "magic" in the newer tube amps (I had the Jolida 502B, which was my first foray into the world of tube amps). This is obviously just me describing MY experience. 5751's are (usually) direct replacements for 12AX7's, and though they lower the gain by around 30%, the right one can sound really sweet in place of a 12AX7.
Have fun, and I'm sure you'll get some really good suggestions here on the forum.