I have been doing this for a long time, and have used prefab, and custom built sealed and ported. It really depends on how much space that one can sacrifice, then musical preference. My best ported was the psi reconed re audio mt15, it was in a kerf ported 3.5cuft at 31hrtz. I have had two different sealed subs that I really enjoyed, a mb quart pwe354-15inch sub in 1.5cuft, and what I have at the present, hst12mkll in .55cuft after displacement, smooth and very articulate, and deep when it needs to be. And it blends superbly with the oem bose system in the car. It may well be that it performs like it does because of the IA20.1, but I am very pleased with it, have to be since I really can't justify getting anything else sub related, I may replace the door speakers on down the road though.