...Sletvana 6550 for AR amps....
Are you talking about the
original Svetlana (Winged C — St. Petersburg) SV6550C, which are out of production now? Or the New Sensor "Svetlana" (Boxed S) ones made at the Xpo-Pul factory in Saratov?
In the former case, I agree (but you have to be careful).
As far as a good tube-gear symbiosis (for my system anyway):Cifte ECC89 and ОРЗЭЛ (ORZEP) 6N23П are the best combination I've found for the Moscode Minuet in A, and the НЭВЗ (NEVZ) gold grid 6Н6П's with 1950's Фотон (Foton) steel-plate GМ-70's absolutely
killed in my Single-Ended Triode monoblocks!
Unfortunately, they were only on loan from a friend, so I usually run "standard" (Ulyanov) graphite-plate ГM-70's and swap in Svetlana copper plates for a capella, opera, or other midrange-heavy content — (at least until I can find an affordable pair of the Fotons).