I don't know your subwoofer specifically, but many people try adding insulation or some kind of foam material inside the box to adjust the sound response. You could try searching other threads here or on Audio Asylum.
Or just buy a REL.
Golfnut, what is the alignment of the subwoofer?
By that I mean, sealed, ported, etc. And, if ported, are you able to be more specific; 4th order, 6th order...
Also, can you give me the dimensions of the box, and any driver specifics?
If you are willing to do some "tweaking", you can get to where you are looking to go.
Aside from where I am going with this, if your subwoofer does not have spiked feet, these will tighten the bass response up a bit.
I hope everybody doesn't attack me for this, but here it goes.
I too was unhappy with the boomy response of my Def Tech PF15. I suspected room modes were responsible. Using my Avia test DVD and Rat Shack SPL meter, I plotted a response curve for everything under 200Hz. It was horrendous, with two huge peaks and very little bottom octave response. Yet, I know the PF15 is a powerful sub and capable of better. I then purchased a pro-audio piece by Behringer called the DSP-1124P, the Feedback Destroyer Pro. Although designed as a feedback killer, it incorporates a digital, 12 band parametric EQ. I put it in line with my subwoofer output jack and subwoofer. I am still fine tuning it, and it requires a lot of patience, but already the results are very promising. The boominess is gone, and my main speakers have opened up considerably. Unlike similar products from Tact Audio that are more automated, but cost around $10K, the behringer piece has a street price of about $130.00. You can find it at places like Guitar Center and other pro-music stores. Other than a very, very slight increase in hum, I am very pleased with the performance and build quality of this piece. So far, it's probably the best $130 improvement I've ever made to my system. I am not an advocate of EQ in general, but in the bass frequencies, it seems to be just the ticket. Email me if you have questions about this.
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.