Thank you for your advices.
StalkerAssassin: I own a pair of 6AS7G too and they're great, indeed. But I like my Sylvania 7236 even more. Maybe I should try to find some other driver tubes for them.
WALL-E: Very revealing info, thanks! As I'm still new on head-fi; did you publish your measurements somewhere or is there any thread/ webpage with a table and general overview? Thanks!