Just those five buttons??? And I thought 220Hz were middle freqs. Seems like it's bass on your amp.
If your guitarists have too much low in their sound and don't wanna change that, then choose a really nasty mid rangy sound. Turn up the "middle" knob to position 7 or eight and turn the volume up. Hit 'em hard then.
For a real bad ass low sound, I would add more 30-100Hz, but you probably can't, so forget that.