If I was you i'd bee looking to spend that 400 on a decent amp then save up a bit more cash for the subs. Either a Genesis Compact 5 or something of that equiv.
With regards to the subs. saying you listen to everything rules out the 15" option as they are only really good a droppin the really low notes, people will hear you 2 miles away but you won't hear much in your car, hehe.
In the polo I have previously, I had 1 JL 12 and that pumped out so much bass it was painful but in the same breath soundered brilliant because of the Control of the JL.
I would suggest 1 or 2 decent 12's, planet audio do a decent 12 at around 200-250 just one of these will rock and drop at the same time.
Remember you ice is only as good as its weakest link and to get the power out you have to put the power in.
Uprate your under bonnet battery and if you can the cables from your alternator and battery, if you can get a bigger alternator and if you really want to run heavy bass get yourself a second battery in the boot (make sure its fused and installed correctly though)