(At first, please read the information for the original schematic.) 

With the switches (S1 and S2) activated you'll change the tune of the filters 
two octaves down (the values of the capacitors are quadrupled). While S1 controls 
Filter 1 and S2 controls Filter 2 you can change the frequencies separately. 

The 47kohm resistors in the feedback chains of the filter OP-amps settles the 
bandwidth (Q), and could be changed to 100kohm or so.

To get a higher output from the filters, I've changed both 10kohm resistors in 
the mixer to the half value, 4.7kohm.
                                                                               
If you activate the switches you will get these specifications:

Filter 1 frequency range: +-3dB, 62.5-625Hz
                                           +-1.5dB, 62.5-375Hz
Filter 2 frequency range: +-1dB, 300-3000Hz