Stereo Upgrade

[kresener]

I'd like to upgrade the stock stereo on my new to me '01 3.0 X5.
Seems I don't even have the sub woofers that I've seen mentioned as being in the right side cubby of the luggage compartment.
Anyone have any suggestions as to how to cheaply upgrade the stock stereo?
Keeping the stock unit is important from a looks perspective, so I'm thinking a sub woofer and maybe an amp? Or would the stock speakers not be able to handle any more power, so a sub and new speakers be the best first step?
Budget is well below $1,000 so while I know I'm not going to have the killer system for that, I'm sure I can get better sound.
Anyone have any budget concious suggestions?