The 6SN7 draws a little more current than a 12AU7, and although I suspect the Mainline's PT can handle it, I don't know for sure.
But the 12AU7 is used for voltage regulation. I doubt very much you're going to hear any difference tube rolling even within the 12AU7 family.
The Mainline does many things well, but its design doesn't lend itself to tube rolling.