There's a lot of ways to skin that cat and most will dramatically affect the tone and feel of the amplifier. Some fairly common options would include;
Pentode/triode mode switching. This basically involves using a switch to strap the screen grids directly to the HT rail.
Fixed/cathode bias switching. pretty self explanatory on this one.
Cathode ground lift on two out of four output tubes. not completely applicable in your situation though. You'll probably want to change output trannies to some sort of compromise value as two tubes would be taken out of circuit.
Built in speaker attenuation with set value. probably the most effective but also the most complex...particularly if you want to be able to use selectable output impedances.
That's pretty much what I can come up with off the top of my head. I'm sure others will chime in on their relative effect on tone and ease of implementation. But this should get the ball rolling at least.