Thanks for replying. I agree, the amp sounds fine with the Loop, Output and Solo switch set to Bypass. I don't understand why the Output Master would function this way. It reacts the same way some "cheap" fx loops do, in that they change the tone of the amp even without having an boxes connected.
If it was me, I would design the Output Master to be at unity when set to 12 o'clock in relation to where the 4 individual channel volumes are set. That way, you could raise or lower overall volume but still have a starting place to check that the circuitry is not altering the true tone of the amp.
So if you plan on using the Road King with any outboard gear, don't bother ever listening to it with set to Bypass. That way, you'll never know what your missing.