Want to upgrade 12ax7's on Jadis OR

First let me say thanks to all the folks in here who have shared their advice and passion of hi-fi with me. Audiogon has become a site that I visit every day... so much great information about the stuff I love most... THANK YOU !
OK . . I have a pair of electro harmonix 12ax7's in my JOR right now, and the system sounds ridiculously excellent to me. I am trying to see if there is a tube that is pretty well undisputed as a marked improvement over my tubes.
I am learning as I go, but I have been reading of Telefunken, Mullards, Amperex, Brimar and some of the other well respected brands, but not sure which would be a better choice over my stock 12ax7's. I'd like to hopefully just make one purchase and be happy with it.
Any recommendations ?
Thanks - Matt