Regarding the Voskhod tubes other reviewers have pointed out that they need a very long burn in period at the order of 120 hours, and that the sound definitively does change and becomes more mellow with less sharp highs.
This has been my personal experience as well, and I like the more mellow sound better. As always, vacuum tubes react differently to different equipment, and each person has to find the combination of tubes that best suits his taste. However, some tubes excel in many setups, and it is a good idea to choose among the best rated ones, and certainly the Voskhods are in this category.
If you like more sizzle at the high frequencies, I have found that the 6N6P-i power tubes accentuate this more than the 6N6P power tubes. Conversely, if the high frequencies are too prominent with the driver tubes you are using, 6N6P power tubes will tame the high frequencies.
As a side note, the best results for me occurred when I used gold grid tubes.