Sub Woofer

[kresener]

I'd like to improve the sound of my non DSP stereo in my 2001 X5.
After speaking with a couple stereo shops, the general consensus seems to be that without replacing the head unit, upgrading speakers would give such a small improvement as to not be worth the money.
Installation of a subwoofer with an amp seems to be my best option.
I know that the more premium sound systems came with a sub woofer in the right panel of the cargo area.
Does anyone have any thoughts on which I should do, go with the stock sub woofer, hoping that the wires for it are already there, or have a bigger sub woofer installed, and live with it in the cargo area?
Thanks for any input.

Kurt