when you goto a concert you don't stand with your back to the stage do you? That's why you have the best front speakers so all the vocals come from there. A good sound stage is where the vocals sound like there coming from high up in the car.
Remember volume isn't messured in watts! Running a mega powerful amp will mean you can run it at 4 ohm easy with the gain turned right down avoiding distortion caused by amps and headunits running flat out resulting in a better reproduction of the music

I'm tempted by a pioneer TS-W101SPL sub which you can get for about ?85 from the right place. 600w and dual voice coils. If possible run your front speakers off an amp too so they get all the power they need that your headunit can't provide. your headunit will also run cooler if you don't use the internal amps.

The better amps are made by Genesis, JL, Diamond, Phoenix Gold, Boston and Rockford to name a few. You get what you pay for in this department and don't be put off by low watt outputs on these amps. They blow the ones halfords sell out of the water!