Thats really one of the beautiful things about this amp. You can swap the power tubes without re-biasing. You do have to be mindful of the impedance, but the switch in the design makes that very easy too.
I like a big bottle in mine, a KT66 or 6550. But I also like them with a vintage 6V6, its pretty nice to be able to swap them with a flip of a switch.
The book describes the various wattages, but I think a single 6V6 is around 4W, a 6L6 is 8-10W. Pretty easy to get the blend of tone and watts you are looking for.