Your not going to do better than what I suggested, thats an awesome high end system for super short money...
As far as car subs, you can easily use a car sub, just get a good class d bridgable amp like this
Crown XLS2500 Drivecore Power Amplifier Series XLS 2500 Amp w Box | eBay
and either rely on your avrs sub output to cross the highs out or purchase an fmod
FMOD Crossover Pair 70 Hz Low Pass 266-252...
But honestly, you are not going to get the results an HSU will give you, not with out spending around the same money, and then you have a car sub in your house... Now if you want to buy a pair of JL W7's and run a pair of mono crowns to them in custom boxes with a real nice eq, and have them auto shop painted or veneered you can end up with something real nice, but still not going to be super cheap-- thats what I would do if I was building a sub, and the cost would be over $3000 {I did the planning but never pulled the trigger}...
If you want to build your own subs
Parts Express.com - Subwoofers subwoofer kits, or boxes, or plate amps, what ever you want all rite there... tried and true stuf, but still the hsu will be better for around the same money..
If you want more options, check out wharfedale diamond 10 series
http://www.musicdirect.com/search.aspx?searchterm=wharfedale diamond maybe some 10.5 towers, with the medium grade center, but you are going to spend about what the focals cost...
also Ascend sierra bookshelfs are nice for your front 3 about $1300 for the set... the focals are less and better imo...
Im sure there are other things out there in your range but, I will put the first set I mentioned against anything out there....