research some makes like alpine, audiobahn, vibe, rockford, pioneer etc.
usually the more expensive they retail at the better quality they are. if you want some low bass your looking for a sub that goes the lowest Hz, all i know about is alpine systems and the type r subs are supposed to be up at the top. ive got a type s in my house and i love it, i just need a better amp. the type s is just below the type r and loads of people have told me the type s and r are sick. pay for a top quality amp aswell itll give you much better bass response and clearer sound. also buy an amp that pairs with a subs power like a 600w sub and a 600w amp it will give you better sound and underpowering is supposed to do more damage than overpowering.
if i was you id get two 15 inch subs or one 18 inch. i remeber my uncle having an 18 in his mk2 golf and it the bass was very, very dark
look on gumtree or ebay theres always bargains on there. dont go in halfrauds either if you dont want to be overcharged on everything you buy.
I spent around £450 (including fitting) on my system in my Focus; 12" Fusion Sub(came with amp), 2 Alpine 6x9s on the shelf, Alpine Component system in the front (6.5" door speakers and tweeters on the dash). Sounds fucking amazing in there, goes more than loud enough without distorting at all. With that kind of budget you'll be able to get an absolute beast of a system. I recommend Alpine for sure!
For that money you can get Rockford Fosgate subs and amps. Fosgate amps are mosfet based and NOT chip. So you will get good deep level bass.
I have a similar system in my car and believe me it beats the bass is tremendous. I dont use a ported design as i like tight bass but even with a closed enclosure type i get bass levels that can put any ported boxie to shame.
Tip: Use a good quality wire. Preferably oxygen free wires. Will cost you more but sure worth it.
Get an Class D monoblock amp supplying 1000-1500w rms through 1 channel, get either 1 15" sub or a couple of 10's - larger subs hit harder, but you don't have as much control over the notes - which are rated to the amp as well. Pay close attention to the ohm ratings. Build your own box out of MDF and make it as big as possible, and ported. For the Subs, look at JL Audio w6 or w7, Re XXX, Soundstream Tarantula etc.
Fuck 6 x 9's.
Decent 2 or 4-channel amp, class A/B and some GOOD components, (Focal, JL Audio etc) to run up front.
Make sure you soundproof the boot and the front doors as well or all that bass will be wasted!
definately dont get 6x9's if you plan on having a sub, the bass from the sub, will interfere with the cones of the 6x9's (usually fitted on the parcel shelf) ruins the overall sound quality completely. Its also worth investing in a quality head unit. (alpine/pioneer/kenwood are some good brands for head units) Im sure someone can recommend some high quality speaker brands.
You will want to invest in a few things, head unit, sub, amp, crossover (for better quality) and component speaker system.
The crossover allows you to high pass the frequencys sent to the front component speakers, and low pass it to the back. So your front speakers will be playing the trebble and mid range, and leave the sub to do all the bass.
As karnage said you will want to pair the amp to your speakers and will require a high channel amp. Or alternatively get one mono amp purely for the sub, and another amp for the component speakers. Pair them based on the RMS power rating of the amps, not max power.
Also dont underestimate the importance of high quality cables and the subwoofer box. Depending on the sub size then there is a specific size of enclosure required and also its to be completely sealed, sometimes they will come with a box tailor made though.
steer clear of sony! their in car audio is terrible, just use their rep to sell it as they make top quality home electronics.