If you're not willing to put your sub off the floor by 6 inches, regardless of the direction in which the sub fires, you should put the sub up on cones or spikes. This will certainly help some of the boominess you have. Another cause of your boominess could be your room, rather than the sub. Try moving the sub around, it might help.
Don't bother just replacing the speaker in it. The cabinet and electronics in the sub were design to make the speaker that is in there. Unless you know all the deisgn parameter and can select a speaker appropriate to them, all you will probably do is make it sound worse.
Before you do anything, try the cones/spike and moving the speaker around. You might find that this will do the trick for you.
