I had some nice NOS 6DJ8/6922's in my preamp, then picked up some NOS 7DJ8's (compatible with the 6DJ8 but far less $) and liked them better. Then got some inexpensive Russian "Rocket Logo" 6922's. They sounded pretty damn nice but still were somewhat rough compared to the 7DJ8's.
Had a pair of the Rocket's cryo'ed and after break in they are smoother and more extended and generally better all around than any of the others. There was a break in period after the cryo though where they sounded not so good.
Sorry to "kick a dead horse", but...
Any difference you hear between the 6DJ8 and the 7DJ8's are probably attributable to the difference in brand and/or manufacture date as they EXACTLY THE SAME TUBE with (slightly) different filament voltages. Do be careful, as the 7dJ8's will not achieve full emission on 6.3V — (it would be like running your 6DJ8's on 5.29V; most likely, not a problem, but in some circuits...), but will LAST longer as a result.
As far as the "Russian rocket" (Voskhod's logo) 6922's are concerned:
Voskhod (and most Russian factories) have two equivalents to the 6DJ8 (or 6922) — the 6N2П which is pretty close, albeit with slightly higher Rp and slightly less Ik capability, and the (preferable) 6N23П, which actually has (slightly) HIGHER current capability than the DJ8's (and a virtually identical plate resistance). If you can find the pre-1980's Svetlana (Winged 'C') version of those, they will still cost less than a quality NOS 6DJ8 and should blow them out of the water.