Really depends what you want.
I fitted an Kenwood active under seat amp in the F which gives a good depth to the sound, cost £108 from ebay and all it needs is a power supply, feed from the head unit and a switch signal lead from the headunit.
http://www.ebay.co.uk/sch/i.html?_od...-SW01&_sacat=0
If you need a lot of bass you'd be better going for a boot mounted one with a separate amp and they start costing a lot more and using boot space.
I know it's not the done thing to have O/E speakers in ones car but I refitted them in the mk2 and get much more depth than the infinitie components that were fitted. (I do have a sub box with 8" subs and a 400 watt amp that will be going in once the strut brace is in).