subwoofer frequencies aren't directional like the higher frequencies. that's why they started putting subwoofers in the trunk years ago (more room, and their out of sight). if you have your system set up correctly, you shouldn't be able to determine the direction the bass is coming from. the phase should all blend together.
actually i had a fiero with a stock sub up front. it was a nice idea, but when i put more powerful subs in the rear (the storage area behind the engine), it sounded much better.