there are no solid solutions for adding to the bose in a cost effiective manner. On the seville I went with a 6" mid-bass driver, 4" midrange and 1 inch tweeter in each of the front doors, 2 6" mid-bass, 2 4" midrange in the rear deck, and 2 1" tweeters in the c-panels. built the crossovers myself, so cabin is 4 channels at 4 ohms, 275 watts RMS each (speakers are rated for 375 total each)...subs I am running 2 2 ohm 12" boston competitor radiator boxes powered at 750 RMS each.
all the way up there is no distortion. so you really have no clue how loud it truley is...get on the freeway at 80, all the windows down and no wind noise....the downside is when you stop you cant hear squat.
Sure you can make it rattle the car, but hey, anyone can hit, some of us prefer to enjoy the whole experience.