VCO makes sense, but I'm more prone to suggest taking that further. Instead of just one VCO, try a Zlob Dual VCO (4 hp) and tandem that with a Ladik O-410 suboscillator (also 4 hp). That way, you get two VCOs for detuning (sounds bigger) AND octave division (major bass) that tracks one of those VCOs. Now THAT would BANG!