I always loved Orion,, still do. It's just that all there stuff is retardedly powered, all SPL and no Sound Quality (SQL). I Bought a couple of 10's and 2 HCCA amps when they first came out,, it blew apart my first car!!
As for SQL i'm a
Focal fanboy. The clarity is just amazing, you can hear tones and notes you've never heard before. They have the best Midrange hands down,, the Treble can seem "tingy" as it's very sharp. Their bass is also good, but again it doesn't have the "punch" that a lot of other subs do.
Honestly i would suggest replacing the Front door speakers with a better set, the rear's are definatly good enough to keep. As for the sub, two 10' subs would be more then anyone needs, i'd go with a Kicker 10's or JL Audio 10'W3v3 subs. Both are a very tight sub and wont need any space at all! A nice 800 Watt amp should do fine,, Orion makes a very nice Class D mono block amps that will murder the Kicker or JL subs.
http://www.orioncaraudio.com/amps/Dmono.asp --
HCCA-D1200 is what you'll need, or you can step up to the next model.
Just to let you know Orion will only say that their amps are "1 0hm Safe load impedance" but i've taken my Orion amp to 1/2 Ohm and talking with my installer he's had one as low as 1/25 Load impedance!!!!! Just nasty.
Hope this helps
Shane