I have rolled in a Cary SLI-80, and also in my Rogue Tempest. You can actually use EL-34 tubes in the Tempest, however the auto bias will be a little hot for them, and they will not last as long, but they're a good deal cheaper than the big bottles, anyway.
Certainly, there is a family sound to a particular tube, but there are so many types of a 6550 available, for example, that you can get a lot of different sounds in one type of tube. My comparison is based on my experience with Svetlana, Sovtek, Electro Harmonix, and Golden Dragon tubes.
The 6550, to my ears, is the "pure" one of the group (if a tube can be pure). Uncolored, maybe cool sounding (again, if tubes can sound cool) not going to have much of a personality, until you've compared to other tubes, at least. Good, tight bass. uncolored (I feel like I'm repeating myself-if that's possible from a tube) mids, and clear, clean highs. I certainly like the 6550, as you may notice.
THe KT-88 tube sounds like a 6550 with bigger balls. Bass is more prominent, not in a bad way, just a tad stronger. Mids are a slight bit darker and warmer, as are the highs. Some might say that it sounds rolled off at the top. I think it is a good tube.
There is a KT-90, I haven't heard them, Kevin at Upscale Audio apparently has some good ones, which, according to most people that try them, are really, really nice tubes. GIving you the bottom end of a KT-88 without coloring the rest of the spectrum.
The El-34 doesn't produce as much juice, but the strengths of this tube are the glorious honey-dripping midrange. Highs sound warm and cozy. Bass is round, not as strong as some might like, and it is a little loose.
I have had nothing but excellent results with Electro Harmonix tubes, they're fairly inexpensive, and widely available. I'd give kevin a shout if you're in the market. There are dealers with lower prices, but he's got the goods, and you know what you're getting.
good luck!!
