It's hard to get a straight story about any differences between Sovtek and E-H branded tubes, but I generally assume they're the same tube with different logos and boxes - after all, they're made on the same line in the same factory, and they look the same when you gaze inside...
In my C-J MV-55, the Svetlanas have been clearly superior, IMHO, to the stock Sovteks C-J supplies: Better extension at both ends, better bass control, fuller midrange, airier highs, cleaner transients and livlier dynamics, with more transparancy and resolution accross the spectrum noticeable primarily as a clearer sense of 3-D space and location, allied to a comparitive lack of "electronic"-sounding artifacts making for a smoother, more natural-sounding presentation overall. In previous posts on this topic however, others have begged to differ (some citing the Tesla/JJ Czech tubes, which placed in between the Sovteks and the Svetlanas in my shootout), so I think it's safe to say that YMMV, and that the particular circuit a tube is used in can make a big difference in which brand is ultimately preferred.
