If the sub has a foam surround - make sure it's in good shape and hasn't started to deteriorate . . . if it has, then get it re-foamed.
I take it that this is a sealed-box type . . . the first thing to try would be to put a bunch more stuffing inside the box - available at Parts Express, Madisound, and others. This will have the effect of making the sub a bit less efficient, and making its rolloff more gradual - this will generally reduce the amount of energy in that boomy region. It's cheap, easy, and reversible.
I take it that this is a sealed-box type . . . the first thing to try would be to put a bunch more stuffing inside the box - available at Parts Express, Madisound, and others. This will have the effect of making the sub a bit less efficient, and making its rolloff more gradual - this will generally reduce the amount of energy in that boomy region. It's cheap, easy, and reversible.