Basically, you want a speaker-level equalizer. Here is one for a car, but I haven't been able to find one for the home just yet... I think google.com would be your friend there. If you really want to, you can always just use that car audio one in your homeby getting a 12 volt car battery charger (or any other high current 12v source) and hooking it up to the power and trigger (give trigger +12v, not ground) leads.