There is only two things that make Bose speakers sound better. One is buy the silver wedge Bose receiver. Two sell it all and start over like Robo said. I would prefer number two. Bose speakers have excellent resale value. You could get real speakers with the money you sold them for.
You're pretty much boned with that Bose system. It uses proprietary connectors; all speakers route through the acoustimass module, which then uses a single, proprietary connector to the "receiver". That particular Bose setup is a closed system, you must use all the Bose products.
And, as others have mentioned, even a modest investment in speakers will yield very favorable results.