DO NOT try pulling two tubes - you'll fry the two in the amp. Like a lot of EL84 amps the Maverick is cathode-biased with a common cathode resistor, so it must be run with all four tubes.
The only really practical way of reducing the power to half or below is an attenuator - the Dr.Z Airbrake seems to work really well with this amp, of the ones I tried when I had mine.
If you never need the full volume you could also replace the speaker(s) with lower-efficiency ones - the stock V30s or C90 are very loud (100dB), changing them for 97dB speakers is effectively the same as halving the power of the amp. Greenbacks would be nice in the 2x12" model, or a G12-65 in the 1x12".