Does this animal exist?

6SN7s have a Mu of 18-20- a bit lower than is practical for a phono section. The 6SL7, which is the high gain octal dual triode, can be a bit tricky as it does not sound like the 6SN7- it sounds more like a 12AX7, and is prone to microphonics in the newer versions.

So these days most manufacturers use the miniature tubes (12AT7 or 12AX7, some use the 6DJ8 family although then you are back to the microphonics) to get the job done. IMO/IME, its easier to build a good sounding preamp using 12AT7s than it is to do it with 6SL7s.

If you are dead set on octals, you might look out for the old MFA Luminescence (made in the late 80s-early 90s), that preamp was all-octal based (although even in the old days, was hard to find tubes for it for the reasons above).

why would you want an octal pre, what is the theoretical advantage?

Here you go. One for sale right here:

Hagerman Octal Cornet

Available as a kit or Jim Hagerman can build you one. it is a well regarded phono stage using the non-octal tubes so I imagine it can be just as good or better with some nice 6SN7s. Only trouble I see is getting really quiet ones.

Great responses guys! Yeah, I've got quite a variety of 6SN7's (phono grade) and wanted to take advantage of this. If it was lacking a bit on gain, I really didn't care so much as the rest of system's gain capability could pretty well make up for it. 6SL7's are not my "direction" so I guess it's back to the miniatures. Say Ralph, do you think that MFA unit would allow 6SN7 / 6SL7 interchangability? Just how big a gain difference are we talking about between these two tubes? Maybe this whole thing is more effort than its' potential gains are worth.