Might be worth loading the amp with 2 cabs in parallel or series, mic one and muffle the other. Should result in a 3dB drop, theoretically.
If you do it in series (as opposed to parallel) then the resistance load should be double (two 8 ohm loads in series yield 16 ohms) and the resultant power/volume should be half. If you do it in parallel it would be half the resistance and twice the power/volume. The changes with load can affect the tone somewhat but whatever.