As far as the subwoofer overpowering the music... that's completely within your control... just don't turn the sub up too high if you're worried about it. Having something with the ability to really pound is good, because it'll sound really good at low volumes as well.
If you're REALLY worried, just buy some new speakers yourself and install them. It's not very hard, and you can read reviews on Crutchfield and buy on Ebay if you're really cheap.
I installed an under-seat amp/sub combo in mine. Doesn't sound nearly as nice as the one you're looking at, but it fills out the music some, and I kept my entire trunk
If you're worried about something being too loud, that might be a good route to go... a little cheaper, more quiet, and great in terms of space savings