6550/KT-88 in a Cary V12r

I don't know the KT77 sound but it is highly regarded when compared to EL34's. Different manufacturers of 6550's and Kt88's can sound very different from each other. ex SED 6550 is very linear from top to bottom but not very detailed. Tung Sol 6550 black plates have that glorious midrange of the EL34 but with a lot more power. Much better base and only slightly better highs. EAT KT88's or original Tesla's (not JJ) are extremely detailed and very linear. The best if that is the sound you're after. Original Gec KT88's are very similar but a little dry sounding. SED KT88's are very nice and musical but a little bloated in the bass. Gold Lion re-issue has better and tighter bass than the SED.

Can't help with biasing. My Octave has Idiot lights that are designed for that amp.

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.

Your experience with the EH KT-88's is similar to mine. When I first used my V12R I was running the stock Ruby EL-34's and they were pretty much "classic EL-34 sounding" with their great mids and fairly rolled off highs with adequate yet satisfying enough bass.I kept bias between 225 -228mA.
Later in its duties I experimented with both the EH KT-88's and the SED Winged C 6550's. Yes, as above, EH's were a bit more hi fi sounding ...better extended at both ends than the EL-34's but obviously lacked their mids...The Winged C's essentially became a very easy to live with compromise...which didn't seem like one after getting familiar with it. Mids were much closer to the '34's in their tangible naturalness...essentially none of the EH hi fi sound yet still did the much more extended frequencies and all with a nicely musical body and tone.
As for biasing I never ran them much differently than the '34's...just under 230mA or so.
For the record I have replaced the amp with a big pair of tube mono's which use the new KT 150's and they do everything all the previously mentioned tubes did best...even better. The KT 120's will fit the V12R too BTW...but you shoulld run less of them in the amp to reduce stress on the transformers... First be sure if you have the beefier ones (call Cary and they will tell you how to look and see). If they are then you can put 4 120's in all 4 of your center amp position (the V12 is essentially 3 amps of 2 tubes per channel L/R) then one more pair in your front amp @ the first positions,(L/R of course) and another pair in your very back positions for a total of 8 tubes...The amp should handle it but you have to reset the biases again too...and those settings you'll need to talk to cary about too...
Just food for thought if you want to get creative.

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.