If your goal is the best possible sound quality, and you don't want to use 12AU7s / ECC82s, then I think you should modify your Crack for the tube type you want to use. For 12BH7s, this means replacing some of the LEDs on the 9-pin socket with resistors. For the 6SN7s, it means replacing the nine-pine socket with an octal socket so you don't need to use an adapter.
This isn't just a matter of convenience, but rather one of performance. I have FOUR 6SN7 --> 12AU7 adapters at home with three different designs from three different manufacturers. They ALL result in unacceptable noise, even when used with 6SN7s that are dead quiet in other amps. And my Crack is dead quiet when used with my various 12AU7s.