Leben cs 600 preamplifier tubes

Yes, I owned a 600 several years ago when the 6CS7 was more readily available. I found slight differences in tone and soundstage, but you really need to have your system placed well and dialed in to make it worthwhile. I liked the RCA best.

I was told by the  owner of Leben that the old 600 could be altered to use 12AX7's (by a tech) but the US importer says it can't be done.  

I have heard the same;  the US importer and service center deny the existence of the modification.  I have checked a few times with the same answer.

Not to answer my own question but rather to leave some useful information for any future seekers of the best 6CS7 tubes for the Leben cs600.

My favorite by far: Sylvania Slant D Getter; great tone and detail. Very rich sounding.

Next best: Tungsol Black Plates: very realistic sounding especially  voices

Third: GE black plate

Fourth: RCA black plate