Start by pulling the two filters, plus a volume/gain pot for each, out of a Series II.
Make sure it's a II, so you get the Continuously Variable Q (CVQ) knobs, rather than toggles with some fixed points.
On the instrument filters, the frequency range is 350 Hz - 6 kHz. Drop the low end down to 45 Hz (this is a Big Deal, especially if you're playing bass).
Add a toggle switch on each channel, allowing you to choose between low, band, or high pass filter mode.
At the very least, add a third knob, so that you can do a 3 way mix of the straight (unfiltered) signal, with two separately filtered signals of your choice. And as Dave says, you can mix them in various other ways.
It's an amazing tool, quite a bit more flexible than what you get on an instrument, and it can also be configured to produce enough gain that you can use it as a preamp, if you don't need some of the other things a preamp might offer (e.g. tube sound, more tone controls, etc).
Plus, you can get one (even new) for much less than Series electronics, though that's not really a fair comparison, and involves some other issues.