system in a hatch (any help appreciated)

First of all, if you're running your speakers off of your headunit, the speakers are definitely not running near their full potential. If you're looking for more bass and even highs, first and easiest thing to do would be to use an amp. Find one that matches the RMS rating of your speakers and that should do good. Also, it wouldn't hurt to deaden your back speakers(maybe even your front speakers too for that matter) to get better bass response.

First thing to do is verify and correct the polarity. The distortion is most likely the result of feeding low frequencies to a speaker that cannot reproduce them. I would add a sub to get the desired bass. To make those Kappa's last longer, either adjust the crossover so that they don't get the deeper bass, or add Bass Blockers to filter them out. The sub will more than make up for their bass output and you will get much better sound quality.

go to thezeb.com and buy some cdt component speakers 6.5" for the door with tweets for the highs and mids.... then buy an amp to power those if u want i have for sale on ebay for $30 that does 100 watts x 2 max and 50 watts x 2 rms... it looks like crap but works well.....

http://cgi.ebay.com/ws/eBayISAPI.dl...&category=39734&sspagename=STRK:MESSE:IT&rd=1

I have kappas all around in mine '93 GTwith a sony head unit, sub and and amp. Like mentioned before you should get a amp (4 channel amp total 600watts or more) the kappas can handle that easy. Also a self powered sub. You can get both for cheap off ebay. I got my 650watt amp for $155 and my self powered Infinity 200 watt sub for $175. I know that's over $300, but not much and it's worth it. Trust me.
Yes the reason you get distortion when turned up is because you don't have enough juice going through, hence the need for the amp. When you hook up the amp, make sure replace the stock speaker wires. I replaced mine with 12 guage. If you don't replace it, you wasted your money getting kappas, plus it could burn up the wire(so i heard). Kinda like getting a intake, cam and exhaust without ever getting new heads or having them ported, it would be a waste of time really. Also when connecting the amp to the head unit, use RCA's and don't run the RCA's along with the power wire. Run the power wires on the drivers side and Rca's on the left or you'll get the whining noise in your speakers when you accelerate. Hopes this helped and didn't confuse you. I had to cut the metal plates to fit my rear kappas also, that was a biznitch, and that's where i grounded the sub and amp, one for both sides and it works like a charm. I'll post a pic of the end result in a min.

I also recommend getting an amp. A nice 4-channel amp: http://www.millionbuy.com/souega4160.html will do fine. If you want better bass response out of the speakers, putting some deadener behind the speaker will help get rid of some of the vibrations, but completely sealing your door will help the most with getting better midbass response.