By most accounts the JJ EL84's are the best in modern production...and they'll definitely sound better than the Sovtek's! IMO the JJ 12ax7's are also the best around.
If you can afford NOS, go for it, but at $15/pair, JJ EL84's are about as cheap as a couple pairs of strings!
Their 6L6's are pretty nice too...though I've also heard the Svet/winged C's are pretty good 6L6's.