To get the best out of your speaker setup, you really should be running a seperat amp(s) for the 8" subs and one for the mids/tweeters.

As Brad said, your setup will show the amp a really bad impedance curve, this will mess with the sound in all sorts of ways. Also, your amp is underpowered for the 8" subs, seems odd I know, but your amp should always be rated higer than the sub your running, this allows for a nice clean signal instead of the distorted signal you get from pushing an underpowered amp too hard (that's what destroys a sub quickly).

Passive crossovers also mess with the sound a lot, you might think it sounds great, but stick in a bigger amp to drive the subs, and just run your mids and tweeters from the existing amp and it'll sound like a totally different system, much clearer and open, you'll get a lot more punch from the subs too. Bad impedance curves can totally change (for the worse usually) a systems sound.