If your head unit doesn't have adjustable filters you need to get yourself down to a decent ICE shop and by yourself some filters. These are basically little capacitors that filter off sub bass. they are fitted in line between the output of the headunit and the speaker itself.
There are different ones available (for differnet frquencies) and what you pick will be dependant on what speakers you;re running and the frequency at which you're running your sub. I would aim to be filtering anything below 100Hz, definitely not trying to produce anything below 80.
