The reason your Bose system was extensive to upgrade is bacause of the way the speakers are amplified. In a bose system, they typically have a small amp integrated into each speaker, meaning there is no central smp powering everything. This means you MUST typically use new speakers and an amp at the least, and usually a head unit as well.

They also use strange Ohm ratings sometimes, such a 6 (halfway between a typical car speaker and a home speaker).

As others have said, figure out what you want to spend and how much you care about fidelity vs. volume.