Agree with above posts, but will add a couple of other suggestions...
You can try to completely stuff the box with foam - the type Sean recommended exactly. While it goes against the conventional wisdom to stuff a ported box, this is what the Fried Line Tunnel or the Von Schweikert Transmission Line(I disagree that it is a TL) do.
Or, you can use polyester batting, loosely filling up the box.
Both of these things can relieve a speaker of its boxiness in many, many instances, turning wheezing bass into more natural sound. As in all cases, experimentation is needed to see if it works for you and/or your speakers.