I owned the Ref2 SE and Ref 5 SE, along with the Ref10 Phono and Ref10. I've also worked directly with ARC on a variety of projects over the years. I've tried different 6550s in different ARC products. While you can roll in any tube that meets required specs, ARC does not care for the original GE6550A. The GE is a fine tube in Conrad-Johnson and Ampeg gear, but it tends to be a bit warm and soft in ARC equipment and ARC discourages it use -- at least they did when I discussed it with them. Their current 6550 that ARC uses is the Sovtek WE6550 and they use that because they can obtain them in quantity.
Prior to the WE6550, ARC used the Svetlana SED 6550C, the "Winged C 6550" (=C=) out of St. Petersburg, and they likely would continue to use it if they could get them in quantity.
People said "Kevin Deal does not reccomend the Winged C for ARC gear." Well that was true, but the reason for it was the last of the Winged Cs out of the factory were of poor quality and the factory had been damaged in a fire. Production ended ~ 2012. That started the meme that the Winged C 6550s should not be used in ARC gear. Totally misleading as those are the tubes the Ref 2 and Ref 5 circuits were designed to use.
Apparently the Svetlana 6550C is back in production by New Sensor (Reflektor) - not the original manufacturer and not the original equipment - but I never tried those or know ARC's take on them. I don't believe they have the famous Winged C logo on them. New Sensor is American owned, made in Russia
If you can find original Svetlana Winged C 6550s, those are the ones to use. People have stashes of them and you can find them for sale.
I've heard people speak well of the original Tung-Sol 6550 black plates in ARC gear. These tend to be very expensive and hard to find.

L-R: SED Winged C, JAN GE6550A, Sovtek 6550WE