4.0 freak for your goals id suggest getting just one better quality 12 such as a RE x.., brahma etc. it makes enclosure design a lot easier, easier to find an amp, and will probably be cheaper in the long run.
as far as that kenwood amp....i did a setup in a friends 3 series bmw using that amp and was fairly impressed with it for a kenwood. Usually when i think of kenwood i think of regular run of the mill amps, nothing too special about them, they do the job nothing more. But this amp was actually worth the money and pumped his sub fairly nicely.
however id say there are better amp deals out there. JBL600.1 or 1200.1 are dirt cheap and put out good power