Your bass? Well, I am not a fan of Bose systems, so my knee-jerk reaction is yank it all out. However, simply adding a sub & a small amp for it would be good.
Add an 8-inch sub if you listen to a lot of rock and country, as an 8-inch is best for tight, punchy bass. Add a 12-inch if you like rap or hip-hop (
), as a 12-inch is best for rumble and drawn out bass lines. I personally prefer a 10, as it is a nice blend of the two. My favorite brand is Diamond Audio or MB Quart, but those are more expensive and hard to find. Alpine is good, as is Precision Power.
Don't be talked into any kind of fancy box or ultra-high-powered amp, as you really won't need either of them if you plan to just add the sub, and you won't notice the difference. Get a mono amp, not a 4 or 5 channel, around 200 watts for an 8, 350 or so for a 10 or 12.