KT88 or 6550 tubes for Rogue 90 amp?

What are the sonic differences between the KT88 and the 6550 tubes in the Rogue 90 amplifier? My friend is ordering the amp Monday and would like to know the difference. I am not sure of the sonic differences between the tubes myself or how they sound in this amp. It will be used with a Rogue 99 magnum pre amp and Theil 1.4 speakers and Acoustic Zen wire.
I would like to throw in EL-34 tubes also in to this discussion. I bought a TAD-60 a couple of months back and just began tube rolling with some 6550 Tung Sol reissues. I had the original Svetlana EL-34's but now have biased and am trying the 6550's. Definitely a difference but, have not formed an opinion yet...

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!!
Gthirteen thank you for all the info. my buddy is ordering the amp monday and needs to tell them 6550 or KT88, I do not think the el34 is an option with Rogue but it is worth asking them about. I was thinking the KT88 usually had better midrange and the 6550 was more extended, actually your description is about exactly what I thought the difference was but my friend had heard the exact opposite.

thank you again

Ask Rogue what they recommend for your friend's particular equipment and tastes. Why doesn't he order a set of BOTH tubes, see which set "voices" better in his system, and sell the set he least prefers?

And remember, it will take time for the amp and tubes to "burn in" before the amp/tubes sound right. Usually, tubes take 75-100 hours to burn in...I don't know how long the amp will take to burn in...again, ask Rogue.

Before burning in, components and tubes will usually sound bright, harsh, edgy, or brittle...it's amazing the sonic change that can take place with 24 hours of play...even though you're not fully burned in yet!