Controlling the bass and treble at the same time is not going to be possible without significant modification to the circuit and another potentiometer. The bass mod also only affects the bass in the overdrive section and not the bass of the whole circuit.
The treble works by forming, on one side, a passive high-pass filter that bypasses the 100K between the last two op amp stages with a capacitor and on the other side a treble cut in the feedback loop of the final op amp stage. There's nowhere in that system to add a usable filter that only boosts the bass without cutting all the treble as well.