Unfortunately, the QS series is unlike many other popular synths in that there is only ONE type of filter... and it sucks!

No lowpass, no bandpass, no cutoff, no resonance... it's a real bummer...

The only way I've seen it done is by sampling four differently filtered sounds and crossfading between them. I don't know about anyone else, but you won't find me doing that...

This is how I solved the problem - I bought a Yamaha as well...

Yeah, I think buying a separate wah-wah pedal would probly be the best way to wah your QS.