in the van it was the 12" hwk, and i used 2 x soundsplinter rlps.
http://www.soundsplinter.com/rlp12_subwoofer_information.htmli have tried mounting the subs in different ways and the volume is about the same, the quality of bass changes slightly goes lower etc).
if your hwk with 500 watts isnt loud eough for you in that room i dont know what would be. adding another sub wont make it louder, but will change the way it sounds slightly, not worth it imo.
just enjoy your hwk for a couple weeks, get used to the way it sounds, try it in different locations in the room, that will make a big difference. try different x over settings etc etc.
after a few weeks you can move the driver to a different location inside the box and then you will know if you prefer the sound of it after moving the driver or if it was better b4.
but trying your hwk in different positions in the room will make the biggest difference.
are you saying your hwk isnt loud?
looks like your amp put out 500 watts at 4 ohms, if your dayton sub is 4 ohms then great, but if the dayton is 8 ohms your probably only getting about 250 watts?
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=300-752