The "distortion" you're hearing sounds to me to be from a square wave. In addition to the octaver suggestions above (all good) I'd try the synth on the GT-10B. Try one of the square bass/dub bass presets on the synth in the FX-1 (or 2) module, and fiddle with the resonance and filters a bit. Keep the cutoff low.
Once you get that dialed in to your liking, you can mess around with assigning the expression pedal to the cutoff, so you can open it up a tad during the higher notes.
Then try blending that sound with the octave suggestions from above, if you're feeling really keen.
