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).
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).