AES/Cary SixPacs - up grade tubes?

I would like to up grade the tubes on my sixpacs, without spending an crazy amount on NOS. Will Electro Harmonix EL34's make an audiable difference, real not preceived over the stock Chinese tubes?
i'm using eh kt-88 and 6sn7 in my cary slm-200. i really enjoy the sound. i've heard that the sed winged c is the best current el34 out there. not my opinion, just second hand info.
I am currently running the Pearl Cryovalve SED el 34's in my Cary SLM 100's, worth the extra money over the regular SED el 34, both are better than the Svetlanas. Haven't heard the EH.
I posed this question to AES (Dennis actually), he said the EH EL34's were an improvement over the Ruby's.

Have you rolled the 12BZ7 yet? If not, go with this first, either with RCA or GE.

I'm using the EH EL34 tubes in my Sixpacs. Sure, I hear a difference, not sure if its better, just different. Maybe it's because I want to, definately not night/day in my system, but better. The extreemes are where I hear it, both highs and lows sound better to me.