There have been a couple of time recently where I thought some extra down the bottom end would be nice, and would ease the load on the main bins. I would run them from the sub out of the Behringer FBQ 3120, through a separate amp.
I have a pair of the following drivers that I am not really using,
http://users.tpg.com.au/users/gradds/jaycar%20PA15.jpg
and was considering using them to provide the extra low end.

Just not sure if they will make much difference??

On the other hand, below 100Hz, a subwoofer placed underneath near the mains is going to sum pretty well, so any additional output will help. If you already have the speakers, it seems like it couldn't hurt to give them a try. Just don't blow 'em, and if they don't work well in this application, you can use them for something else.

I will probably try the Jaycar PA15s as some bass boost just to see if it works. The other option is buying some new 15" drivers, but I'm on student wages at the moment:-((

Those Eminenece Magnum HO's look rather nice in a smallish box !!

You might look at the Omegas, as they sound good and cost a lot less than Magnums. No shorting ring, but they sound very good, better than most without flux stabilization. I think it may be because they make the static flux symmetrical.

Id be more interesting in
-System sensitivity
-Boundarys