A preamp really means that the pickups run through a buffering and/or gain changing circuit which may include active bass and treble (and sometimes midrange) controls. EMG active pickups have built in preamps, which is why the pickups themselves require battery power.
As for expanding your options, you could go a several ways...
If you are working with three knobs, you can't use a BQC system since it requires 4 knobs.
A quick note about EMG's product naming scheme: the EMG "systems" include a volume and a balance control, while the EMG "controls" do not, otherwise, the tone shaping circuitry is the same for a given type.
You could get a BQC control (
http://www.emginc.com/products/index/105/146/4) that has a concentric Bass/Treble stack (the BTC essentially) and a concentric midrange stack (the VMC) and then use a stacked 25K ohm pot for Volume/Volume. I bought one (complete with knobs) on eBay for a reasonable price. Something like this one:
http://cgi.ebay.com/EMG-CONCENTRIC-G...item3cb3ed815a
Or, if you didn't want midrange control and wanted a balance control or two separate volume controls you could go with the BTC (stands for Bass, Treble, concentric) system:
http://www.emginc.com/products/index/137/146/4
Or, you could do the stacked Volume/Volume pot, a BTC control (
http://www.emginc.com/products/index/140/146/4) and a Bass Expander (EXB) for on-board contour control (
http://www.emginc.com/products/index/136/146/4)