Audio Research amp's retubing experiences

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.

I retubed a VT200 with SED 6550's from a well known Canadian tube seller. One of the tubes shorted upon turn-on and this blew out a resistor and a few inches of circuit trace. My savings cost me $500 in shipping and repair (The 200 must go common carrier). What is the liability of buying a bad tube from another seller? They replace the tube. That doesn't come close to my costs. Also, pins on these replacement SED's were thinner than the ones on the ARC SED's, making a much looser contact. That may have been the problem, not sure. If you retube through ARC, it comes with a warranty. If you can test the tubes other than in your amp, then save the money and retube elsewhere, but make sure you are getting the thicker pins.

Dbarger, were your replacement SEDs in the Rew/White/Black Winged C box?

When I owned my Ref 600 Mk lll's I retubed with KT 88's which produced much sweeter midrange but at the expense of bottom end. Nonetheless, much better result than the 6550's provided by ARC