Don't focus too hard on the wattages. 400 watts and 1000 watts are not that far apart. E.g. to double the volume in theory, you have to increase the wattage about tenfold! So theoretically, more watts = more volume, but between 400 and 1000 watts you will have almost no difference. Also, it depends much more on the design and quality of the components than on the wattage. I have a 50 watt system and could bring down the walls with it ;-). Watts are, by and large, just marketing gobbledygook, unless it's about matching amps and speakers, in which case it's important.