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.