i think you guys have overlooked a very important issue,how much power does he have?
Sure you can get much better subs and get louder but it really wouldn't make that much difference if he doesn't have the power or proper elec. to run those bad *** subs anyway? So basically if you plan on upgrading your subs make sure you have the power to run them,also the efficiency of the spkr. makes a huge difference in how loud they'll play with a given power/box combo?