I just put NOS 7308's in my VK-30 about two months ago. It took a week for them to really smooth out and open up. Bass also recovered somewhat, but it's still not as tight as with stock Sovtek's. The highs and mids, however, are really glorious--so open, so smooth, so much air and dimensionality to the music.
By the way, where did you get them and, if it's not too much to ask, what did you pay? I searched a long time and ended up paying a lot for three matched pairs--$800. But then, I have also paid almost a grand for a couple of three foot long wires that just sit there passing a signal. The tubes are an active device, which makes them far more important to the overall sound. You'll see--just wait a bit and keep 'em playing, or at least in standby mode.