Fact is that the IS actually has 8 speakers. The mids and seperate tweeter in each front door, and the so called 6X9 coaxials are actually a 6X9 WOOFER with a seperate mid/high mounted with in it. The factory amp sends the lower frequency's to the 6X9 and the upper range to the seperate speaker back there. In terms you could say that each speaker is just one speaker, but it actually is driven by its own amplifier channel. This is why so many people say that when they changed the speakers it sounded like ****. If you get a stealth component 6X9 like Lanzar(Not endorsing, I just know that these are this way) they have seperate inputs on the speaker for the low freqs and the hi freqs. This will allow you to change to an aftermarket speaker and get more than just the high frequency's, or just the low's. Also, please note that the woofer is running in the factory system at 2 Ohms and the mids/highs are running at 4 Ohms. That means that the woofer portion is receiveing at least 50% more power than the highs. Make sure that if you upgrade you replace the speakers with the same impedance. If you want to increase bass without losing the trunk here is a simple idea, but its not the cheapest.
Remove the factory 6X9's, purchase a pair of USD audio Pro 6's. These are a 6" subwoofers(Yes an actual sub) that handle good amounts of power. These will set you back about 200 a piece. They are worth it as these 6" speakers can produce the same 40HZ SPL as 2-10" subs mounted in a ported enclosure. Also purchase you favorite brands 3.5" speaker. Make a mounting block out of wood that will hold both the 6 and 3.5 speakers and mount to rear deck using factory hardware. Use the factory amp's high rear out put and connect to the 3.5's. Use a linelevel converter and hook to the rear low freq amp outputs. Install a good amp that can deliver at least 300 watts mono and connect to the 6" speakers. If properly done the amp can be mounted under the rear deck as it will not take hardly any valuable trunk space. Using this combo is the equivalent of installing two 10's. You will have much better low-end extension, and more basssssss! This is only 1 or I'm sure 1000's of methods, but I just did this combo on a customers car and it absolutely blew his mind.