Yeah, I don't disagree... gain is often a problem with effects, particularly with active basses, and it may well be the problem here. But some people assume that it is the only possible issue if an active bass doesn't play nice with an effect, when in my experience some combinations simply just don't work.
True- the Z-Vex Wooly Mammoth is infamous for sounding like crap with actives, so the Vex Mastotron just added a pre-circuit pot that helps with the input impedance. I think the Gain knob on the Oxide works in a similar fashion so it can work well with active instruments.