Audio Research amp's retubing experiences

I recently re-tubed my 2 ARC Classic 120 monoblocks. I buy from the Parts Connexion, 16 6560c Svetlana`s, 20 tubes total for about a reasonable $450, (I requested and got matched quads).
I too was going to try KT88`s but at the last minute I found out you have to move a wire in each socket if you change to KT-88`s, something I can`t do.
This is my fourth set for these amps and I`m very satisfied with the Svetlanas, your situation with a ref 600 may be different,
Len W

I didn't like doing it, because it was expensive, but I worked through my dealer and re-tubed via ARC for my VT-100 MkIII. I'm glad I did. In the process, they found a bias adjustment problem, and did a minor repair that will prevent unpleasant occurences in the future. Also, the end result sounded as good as new. (I think tube-rolling is kind of interesting, but, having blown up several tube amps over the years--which I've found to be ugly experiences--I leave the amps to the pros. Pre-amps and phono-pre's are a different story...)

Just my two cents.

The most recommended tubes for the Audio Research amp are the 6550c SED. I retubed in the past Classic 60 with Svetlana very successfully. I will retube a pair of Classic 120 in closed future and i will use 6550c SED tubes.
I know the telefunken E88CC and ECC88, both work very well on Audio Research Preamp, but inside an amp like your pair of Ref600, to be honest i don't know. Be aware that the telefunken tubes are starting to be very expensive, no way to get a pair of E88CC fully tested for less than 200$

I don't think you can go wrong with the 6550 SEDs. There may (may) be a better NOS, but that can lead down a slippery slope that may not sound better with your ARC than the SEDs. I would look to Upscale Audio or the Tube Store for good matched sets. If you are still interested in KT88s, I would ask ARC for their opionion - my sense is their circuitry is optimized for 6550.