I recently picked up one of these and am not regretting it!
I'd been searching for a decent, highly-tweakable bass synth pedal since my GT-6B* went kaput...
If you're lacking bottom-end there's a few options...
( a ) [b]increase the amount of "dry" signal[/b]
Connect the pedal to your computer and, with the MarkBass software, turn the "mix" knob anti-clockwise.
Roll-off your instrument's upper frequencies using its tone knob / on-board EQ
( b ) [b]use an octaver[/b]
Try shoving a Bass Octaver after the synth. Note that very few octavers that will actually be able to handle this without going schizo!
But if you have a Line 6 M5/M9/M13 you're laughing as it tracks like a beast...
Even so, this will give really muddy results if the Coarse Pitch of any of your oscillators is set below 0, so be careful - many of the synth presets already include an oscillator pitched at -12 steps (sub-octave), so your extra octave will come out at a bowel-troubling -24 steps(!)
( c ) [b]detune the oscillators[/b]
Set up two oscillators with the same Coarse Pitch (e.g. 0 or -12), and vary the Fine Pitch.
Don't be afraid to go a bit wild! Greater detuning will give more DnB-style tones.
In general, I'm running the synth through a shedload of other FX anyway - MF-101, overdrive, flange/phase/chorus/etc, delay/reverb... if you want it to sound like a synth, treat it like one!
[size=2]* the synth on the GT-6B is still the most tweakable I've come across, once you get used to the menus and assigning parameters to the CTRL and EXP pedals... just wish it had more than one oscillator... and was still in working condition!![/size]