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 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$