If you look at the Torii MK4 tube rolling thread it is discussed.

My limited experience is that they can change the sound significantly to radically.

You can play with the power tubes 0_3 tubes octal and the inputs 0_2 tubes 7 pin.

For the 7 pin there are some other variants like the 75C1

Tip for searching search for both "o" oh. and "0" zero when searching

Example: "RCA 0A3" then another search for "RCA oA3"

I recall Will going into extensive details about his findings some where.

Brad, I think you'll find that the amp ships with an 0A3 tube. . . the "OA3-OD3" I think is meant to indicate that all four of those types (the "0" designation is I think more accurate than "O") can be used to regulate the power tubes and each imparts a unique signature, and different brands and types of each (straight sided short and tall, bottle-shaped, in the case of D the "W" ruggedized type) will each have a subtle characteristic of their own. Rolling these can definitely influence the sound of the overall amp. You'll also likely receive an 0C2 regulation tube for the input tubes, and you can use 0A2, 0B2, 0C2 and a few variants again in different brands to fine tune the sound of the amp.

These voltage regulation tube spots and the power output are the two advantages of the Torii type in my regards. I personally, after living with Toriis for almost a decade, and with SEP EL34 amps from Steve as well, prefer the Zen and Taboo amps with their small power tubes for sound character. I love the voltage regulation tubes in my SE84UFO3 Monoblocks and if Steve made a Taboo with these as well I'd snap one up for my system in the living room as well -- what an amp that would be!

Reading this forum over the years most people are using 0A3 and 0C2 in their Toriis though the last few years with mine as it had seasoned in and I had spent years beginning to understand the amp I moved to 0B3 and 0D3 and to the 0A2. I would recommend running the amp in with the supplied tubes and not worry about rolling those spots until the amp has been months in use.