Did the same thing on V2 .. replaced with a MESA 12ax7 ... more low end, sounds "heavier".
Tried a RUBY brand 12ax7 in V2 and it sounded frigging horrible.
Had previously tried the MESA in V1 and that sounded horrible.
So ... it seems like .. if you want to play .. play with V2 (12AX7 on the left).
SOVTEKs are know for being transparent ..... I assume MESAs are "built for modern, heavy gain."