OpAmp Ne5532
This is a bass filter, using the OpAmp Ne5532 or another of your choice.
The circuit has two pots, one for volume adjustment and another to adjust the filter cutoff frequency of 50 to 150Hz, if you need to install the pots in a panel, use shielded wires to make connections.
The circuit is simple to set up and already has symmetrical power supply, the transformer must use 9-12 volts / 500ma with center tap In many cases the power source can be made by the power amplifier Just watch out for the limit supported by the voltage regulators.