The recently announced PLAYdifferently Model 1 hardware mixer (http://playdifferently.org/ ) features an interesting and innovative new system instead of the conventional 3 band EQ found conventional on DJ mixers and DJ software. Each channel has two knobs for its "SCULPT" EQ:
'SCULPT is a semi-parametric swept bell filter, and consist of two controls,FREQ which sets the centre frequency and CUT/BOOST which adjust the gain or a attenuation at this frequency. The level of cut/boost is asymmetric; maximum boost is limited to +8dB, while the cut is extended to -20dB. Sculpt has a wide Q and a frequency range that covers almost seven octaves.'
plus two knobs for "contouring" LPF and HPF:
'As the name implies these two controls allow you to “contour” the frequency range of the audio signal. They consist of low Q, High-Pass and Low-Pass filters designed not to add colour to the sound as they are swept across their frequency range.'
With Mixxx's EQ effects system, this kind of setup could be used with any DJ controller with the conventional 3 knobs for EQs. It would require having the LPF and HPF combined into one knob like the are for the existing filter effect's superknob. The other two knobs would be used like the FREQ and CUT/BOOST knobs on the PLAYdifferently Model 1. For controllers with the conventional 3 EQ knobs plus a filter knob, a different version of the EQ effect could be provided without combining the HPF+LPF into a superknob.
I don't think it's possible to combine the LPF and HPF filters in this way because the goal of the Model1 is that the DJ can engage both filters at once to limit both the high and low frequencies. With only one knob, one filter has to be unengaged while the other is active. But, we already have code for four-way eq (though it is not well-tested), so we could support this eq style that way.