lol, yeh, Evil Empire sounds sh*t in mono
IMO you don't need two or more valve amps, but you do need a few pedals. Blending a bass overdrive with a guitar distortion, and plenty of control over the EQ on both channels seems to work well for me. Different compressors on each channel can be great too. You also need plenty of high mids from your rig. This might mean you need to add a guitar amp - my peavey 15" combo would never produce enough high mids for this sort of sound.