You're current set up seems great. I've heard those speakers before getting the same power you're putting to them, and I loved the way they sounded. But all that matters is that you enjoy them.
If you're going for more sound quality than bass in yo face, this is what I would recommend:
250W 10" focal sub - $200 -- more info on the sub
smaller box - $70 -- bigger box - $80
I would recommend building your own box if you have the time, tools, and hand eye coordination. I say the last part because my buddy almost lost a pinky building his box, and I don't think a sub box is worth a pinky.
If you're going for sound quality, I'd set your HPF/LPF on your head unit anywhere between 60hz - 100hz depending on how you like the sound of the lower frequencies coming from your door speakers.
Edit: wire the sub like this.
and having this set up would make my OCD happy just because the amps are the same brand and the speakers are the same brand.
Edit 2: you're also going to need 2 of these RCA splitters ($4 each) to plug into the top RCA ports in the back of your head unit for your front and back speakers so you can plug the RCA's for the sub amp into the bottom set. You'll lose the ability to fade sound from front to back (not left and right). If you really want to have the front/back fade ability, you'll need to upgrade to a head unit that has three sets of RCA ports in the back.
