Enclosure might make a slight difference, yet the amount of money and time that you put into it. Your better just getting more equipment. Again, the aftermarket speakers that are sold are designed for mid-range and highs, try as you might they are designed to reproduce at certain frequencies, so the lows are unlikely to get much deeper.

 

if your going to get a sub theres no need for any modification of the speakers at all, i would just leave them as is