Hi Mjcmt,
I don't know if one can reasonably generalize about even a personal preference on type of tube based on a single comparison. First, type of tube chosen would depend not only on the designer's preference for the sound, but also circuit requirements, amount of gain needed, etc. The best tube is one that best fits the requirement of the component. Also, 6SN7s, like all tube types, have quite different sounds, depending on the specific tube chosen.
I have some familiarity with Cary triode amps (not the pentode EL34). They can sound dramatically different, depending on the 6SN7s used in the amp. Quite often, the 6SN7 chosen by the manufacturer is picked with cost and availability as primary concerns, not necessarily "sound" first (this makes sense because the amp has to be reasonably serviceable, and because "sound" is a personal thing, why put a lot of money into an alternative that may or may not be considered better by the buyer?). I've heard great improvement with alternative NOS 6SN7s used in their amps. Have you tried alternatives?
Realistically, one does have to audition stuff, with what comes from the manufacturer for the first "knock out" round of auditions. Unfortunately, that may mean eliminating a particular piece even though a better choice of tubes might have made it a contender.
If I were at a dealership that offered a preamp that uses something other than 6SN7s, I would still at least give it a shot, because, when properly used, the results can be terrific too. My phono stage, which I think is killer good, uses 12AX7s, but, I had to try quite a few different types to get to the ones that work the best, and sonic differences between types were substantial.
