Haven't been in here in awhile, but thought I'd weigh in (BTW, the Sound Shop forum is alive and well, contrary to popular belief: it's just that the default viewing option displays only one day's posts and I think a lot of good discussion is lost because people think it's dead - change the "display option" to a week).
Anyway, a couple of comments: first of all, full frequency range is sent to the rear deck speakers, it's just that the stock speakers really suck. Before I disconnected the factory wiring I hooked up a pair of coaxes with the factory wiring going to the crossover inputs and yes, poor quality amplification, but full range of frequency. Immediate improvement to be had by just replacing the rear deck speakers with two- or three-ways and nothing else. Standard 6 x 8's or 5 x 7's fit effortlessly (o.k. not so effortlessly because those mounting screws are a bi*ch! lol). Next, front doors are just not the place for subs; there's no room for the air displacement necessary to create good bass. That and the fact that most posters have or plan to have sub(s) in the trunk means just don't bother trying to find a replacement sub that fits: use the space for a decent, smaller-sized mid (6 - 6 1/2" can be more easily made to fit and will produce more than adequate bass for the front) or just disconnect the dam*ed things. And whatever you do, don't change the head unit and try to make it work with the existing speakers and amps - you may, eventually, get it to work but it's just not worth it. If you're going to invest, do it right and do the whole thing. Besides, as far as stock audio goes, the Shaker 500/1000 really isn't all that bad... IMO see you in Sound Shop!