Want to upgrade surround sound

I have a Harmon Kardon AVR1700 reviever and Bose Acoustimass 10 speaker system installed in a large room (33x19). I'm not satisfied with the low ends. After reading a few articles, I've found that the Bose speakers are not very highly thought of.

Is there anything in the $400-$600 range that will give me good surround sound speakers and a descent sub-woofer?

Another question ... can I just ADD another subwoofer? I know I need to keep the Bose sub because it's a crossover for the other speakers.