G'day people I've got a VS Berlina with 4 factory 6" speakers and a PIONEER DEH-P5750MP. But best of all i have 700 bucks to spend on an amp and a sub i was looking to get a better quality amp [up to 500 bucks...] was also thinking of getting a 2 channel amp as in the future i was gonna get 2 subs...}
If your budget is $700, then spend more on the sub side. You can have the best amp in the world, but with low quality sub, it won't sound any better than $5 speaker. If your setup is daily driving, then give JL 10W7 in a HO box a try. It's a bit compact in size, yet hold a big punch.
Are you looking for SPL (loud), SQ (sound quality), or SQL (the best of both worlds)?? If you answer that we will be able to suggest something for you.