You'd probably get best results by adding an amp to your components and running a dedicated passive crossover like this one:
Soundstream Exact 4.3 
The problem is the very low power you're sending to your components. It's actually around 15w RMS/continuous per channel (the peak power rating of 45w is a useless measure that's purely marketing jargon). Swapping in components with a lower sensitivity will only make it sound worse.
Crutchfield still has this nice
JBL GTS180X for $80 that would really help your components.
There are also tons of good 2ch amps you could pick up off of ebay for much less than $100. I picked up an new MTX Blue Thunder PRO502 amp for $50 a couple of months ago.
