That's an interesting question Pubul. I to have been told with my modded JH 50 that most other tubes besided my current JJ's will sound mushy, slow, or lack detail. This will probably not stop me from trying some out though as i ascribe to "in your system to your ears" theory. Plus learning about how my system reacts to differing changes is good to know as well!
I would enjoy though an answer to your question if anyone is willing to expand.