Oct. Combi: Bass output

[zzonyx]

I'm considering two alternatives to get decent bass out of the stereo without sacrificing ANY trunk space: replace the original rear loudspeakers by a better pair (which ones?) or adding a subwoofer (my stereo has the sw output as standard). Which one will it be depends on the available experience of whoever responds.
Is there a ready-made, not too big subwoofer solution complete with a box and all the electronics that fits into the spare wheel rim in the trunk?
Kindly supply info and/or links.
Suggestion and/or photos on how to make the right holes for the sound output and keep out the dirt and other unfriendly objects will be welcome too.
Best regards to all.