Raise mids and lower the bass. As the volume (Gain) comes up, the low end will get phatter and thicker so lowering the bass will help compensate. The humbuckers are providing more output which is driving the preamp harder, which is probably somewhat responsible for the low end getting tubby. ALso, if the humbuckers naturally produce a lot of bass this will have to be accounted for.
The tone stack on this amp is a particularly powerful tone shaping tool. The fact that the mid knob also controls high mids / low treble means that it's possible to change the voicing on the amp in a fairly dramatic way. Lower mid settings create more of a recto - esque scooped tone, especially if you open up the presence which reduces negative feedback in the circuit. On the contrary, raising up the mids will create more of a marshall - esque crunch tone. The balance between Treble and Presence can then be adjusted to fine tune the sound. Even some more extreme settings can give some really pleasing results.
