This circuit allows you to adjust the frequecy at which you would like to boost the bass. This is good for certian rooms to get the effect you want. It adjusts from 35 Hz to 150 Hz and has a boost and cut of +/- 12 dB. You need to use low noise op amps for this so that it won't interfer with your siganl.. To get a better look and explanation of this and some other circuits you can go to a great page ESP Audio Project Pages.