Jwin: I'm not sure, but I think ARC currently supplies Russian 6550s, probably Sovtek(?). They in the past have been good about checking out the tubes they sell for quality and in matching sets, so there's an advantage to using them. If you haven't retubed for a long time, you may be replacing Chinese tubes, which in my view will result in an improvement with a new set from ARC as well.
When the Classic 150 came out, I converted my M300s to Classic 150s, and used Phillips 6550s in them, which is what ARC was using in the late 80s to tube their amps. Those sounded excellent to my ears, much better than the Chinese tubes ARC started using about 1990 or so. Lasted about 5 years in my M300/Classic 150s before they started to sound a little dull. I still have a ton of those tubes, as they sounded great in my Jadis JA80s as well (which replaced the ARC amps in my system) when I ran those amps full range, so I stocked up on the last ones ARC had available (I now use NOS KT88s). I'd say they sounded similar, in the Jadis, to the Svetlana 6550C's that I also used, although I still preferred the Phillips as a little sweeter sounding and cleaner. So that's one possibility for you, although I'd say it's a close call sonically with the Svetlanas, and in the ARC circuit the Svetlanas might sound better. You could also try some NOS Tung Sol 6550s if you want what many, including me, consider to be the finest 6550s ever made, both sonically and for longevity, but be prepared to pay top dollar and use someone who knows their tubes as a supplier, as there are some cheap imitations floating around out there.
Another possibility, which would probably require a return of the amps to ARC for rebiasing (and at their weight, a good deal for shipping), is to use KT88s. When the Classic 150s first came out, I recall Jack English reviewing them in Stereophile and far preferring them with KT88s; a little richer midrange, less of the "whitish" sound. I think ARC discontinued using KT88s after a while for reliability concerns, as the Chinese KT88s they were getting did not hold up very well (I recall them telling me that when I inquired about changing from 6550s to KT88s). With other KT88s now on the market from Russia, perhaps this would be a viable option. You could ask ARC about that.
Hope all of this makes sense and helps; you've got some nice amps there, take advantage of them. Good luck!
When the Classic 150 came out, I converted my M300s to Classic 150s, and used Phillips 6550s in them, which is what ARC was using in the late 80s to tube their amps. Those sounded excellent to my ears, much better than the Chinese tubes ARC started using about 1990 or so. Lasted about 5 years in my M300/Classic 150s before they started to sound a little dull. I still have a ton of those tubes, as they sounded great in my Jadis JA80s as well (which replaced the ARC amps in my system) when I ran those amps full range, so I stocked up on the last ones ARC had available (I now use NOS KT88s). I'd say they sounded similar, in the Jadis, to the Svetlana 6550C's that I also used, although I still preferred the Phillips as a little sweeter sounding and cleaner. So that's one possibility for you, although I'd say it's a close call sonically with the Svetlanas, and in the ARC circuit the Svetlanas might sound better. You could also try some NOS Tung Sol 6550s if you want what many, including me, consider to be the finest 6550s ever made, both sonically and for longevity, but be prepared to pay top dollar and use someone who knows their tubes as a supplier, as there are some cheap imitations floating around out there.
Another possibility, which would probably require a return of the amps to ARC for rebiasing (and at their weight, a good deal for shipping), is to use KT88s. When the Classic 150s first came out, I recall Jack English reviewing them in Stereophile and far preferring them with KT88s; a little richer midrange, less of the "whitish" sound. I think ARC discontinued using KT88s after a while for reliability concerns, as the Chinese KT88s they were getting did not hold up very well (I recall them telling me that when I inquired about changing from 6550s to KT88s). With other KT88s now on the market from Russia, perhaps this would be a viable option. You could ask ARC about that.
Hope all of this makes sense and helps; you've got some nice amps there, take advantage of them. Good luck!