6550/KT-88 in a Cary V12r

In the end I got a dozen Electro-Harmonix 6550s. I was advised to bias the amp a little higher than with the EL34s or KT77s, around 270 mA. They certainly have a little better bass control, and comparatively they're very quiet. They're a bit drier sounding than the KT77s/EL 34s - more linear, I suppose. They don't have that same midrange richness, but they've got better extension top and bottom. Unfortunately I picked the EH tubes because they're inexpensive as 6550/KT88s go, and they're supposed to be pretty rugged. I don't expect them to be the last word in tubes and I'll likely revisit the issues in a few months, as funds allow. I understand that some of the more expensive 6550/kt88s do a better job with the mids.

The amplifier is running just fine with them installed. Temperature seems consistent with the old tubes.

just to follow up: The EH 6550s have been in my amp for a month or so and the sound has changed for the better during that period. I don't know if it was the tubes burning in or what, but the bass has become appreciably tighter and more detailed than when I first installed them. Acoustic basses have much better definition and texture, and even a little slam, which is not the V12s forte. The mids have opened up a bit too, though EL 34s are still champs on that score. I also find them to have quieter backgrounds than either EL34s or KT77s, although the transformer still hums a little and there's still some mild hiss up close at the speaker (the joys of p2p tube amps). When I bought them I thought of the EH tubes as a compromise for price because I was in a pinch after unexpectedly blowing a KT77. They really are about the cheapest 6550/KT-88 available, but they sound quite nice in this amp. No hurry to run out and buy more expensive tubes. Hope they're more reliable than the JJs.