I had 2 iffy Sovtek 6550s in my ARC power amp. I replaced them with what I thought were well priced NOS Sovtek 6550 from Upscale.
Now I am not an absolute expert on Sovtek tubes and their iterations throughout the eras but I do recognize obvious differences. In my case the Sovteks may or may not have been NOS but they certainly were NOT Soviet era tubes as my originals were. The newer tubes were noticeably lighter, thinner materials and apparently different metals at the base.
I learned another lesson- relace ALL the tubes not just the weak ones.
And another lesson: ARC matched tubes are in all respects superior to any I have encountered in the aftermarket. My matched set woke up my amp in a delightful way. More volume was the first surprise-- then the depth and detail presented in my favorite test music. It was a real joy to rediscover my music and my faithful old friend and room heater from ARC.
My local dealer let me have the ARC matched set at his cost. (He recently sold me some Maggie 3.7x so I had some good will leverage)
My final conclusion: I got what I paid for- plus paid a stupid tax not going top shelf first.