6550/KT88/KT90 variants in Audio Research VT50/100

There have been some posts here on the subject but nothing very complete. I just bought used AR VT50 and enjoy it alot (and am considering getting a VT 100 Mk2 or 3 soon). I have read that the SEC 6550 is used by AR and is a good one, but from my experience with 300B Single Ended amps I would like to consider some power tube rolling in these amps - would appreciate thoughts on the other available variants (Sovtek 6550, ElectroHarmonics KT88, etc etc) and how well they work in the ARC amps... thank you in advance...
I retubed recently an Audio Research VT200 with KT88 EH without any issues. I used to have also good experience in the past with Classic 60 amplifier (i had 2) with Svetlana SV-6550c and the other one with 6550 EH.
I heard also that the Svetlana (SED) KT88 are working very nicely as well, you should be able to plug into your VT50 with no issue