Im not really sure if two 15s will fit in the trunk of my 04 grand am.. but either or im going to need an amp to power the 12's or if possible the 15;s
Nate if you want a box size that wont be too large go with two 12's with the amp that you have step down to the audioque sd 2s and give them the 500 watts at 2 ohms your amp you already own produces. That way your getting more cone area like it seems you want and keeping the cost low. It would cost you $268 shipped for two of those 12's. Unless you dont car about the space the box takes up and want to spend a little more to go with the 15's in the sd2s. The 15's would cost you $308 shipped for two of them.
For one or two hdc 15's? you better have about 7 cubic plus to run two of those 15's and a decent electrical upgrade otherwise id stick with one hdc 15 (or possibly an 18 if you have the room) and run an audioque 1200d to it. The 18 is the way to go if your looking for the most cone area on minimal power because one 18 has the cone area of two 12's. I dont know what box size they need but id guess 5.00-5.50 cubic feet. You could call DJ at audioque and ask him hes a good guy and has great customer service.
scratch that, just found out im not getting the job.. that blows, but im still just gonna buy a 2.5 15 and attempt to make a box when i get some more money