You don't *have to* replace the tweeters in the pillars. I ran all new wiring from my Alpine amp in the back to Alpine Type R 6.5"s in the doors. Sounds great despite the absence of tweeters.
The Bose speakers have a lower impendance than most headunits are rated for. In addition to sounding like crap, you risk damaging the amplifier(headunit).
You could get an amplifier adapter. Something similar to this:
http://www.crutchfield.com/S-0...apterI've never used one of these so im not sure how it affects the sound quality.
In my opinion its well worth it to save up a little more $ and get some good quality speakers (and an amp)... and subs...