The switch determines what mid band you're boosting/cutting. So if you dial back the mids knob to say, 9 o'clock, and have the mid frequency switch all the way to the right in position five, you're cutting the high mids. If you have the mid knob at 3 o'clock, and the selector is all the way left, you're boosting the low mids, just above the bass range. I forget what the exact frequencies are at this point, and I'm too lazy to look them up.
I've had mine for a while, never had any problems with it. I ran it with a 8 ohm 2x12 and a 4 ohm 2x15/2x6, worked fine either way. I'm looking into getting rid of mine, just as I personally don't like the tone of it as much as I thought I would once I got it in my band context(I play in a room with horrible acoustics, so YMMV). One thing that worked well for me was the volume knob was gentle up until noon, as I need to be at a fairly low volume most of the time. I'm using a Kustom 200 now, and that is pumping out about as much volume wise as my PF-500 did.
