New items arrived at my doorstep this afternoon!
A Precision Power FRX-456 crossover. This is quite possibly the most robust stand-alone active crossover I have seen. I researched for over a week trying to find a cheaper alternative that would serve my purposes but I kept coming back to this. I ended up paying $110+shipping from eBay. I got screwed by one of those second-chance offer things (I should have only paid $90) but I wanted it now and there weren't any other auctions except for buy-it-now for $160.
Anyway, the crossover is capable of 6 pairs of outputs. Subwoofer, Front high/mid/low, and rear high/low. I won't be using rear speakers so that opens up some options for me. Specifically, I can use the front 3-band PEQ of my head unit for mids and highs and the rear 3-band PEQ for lows and sub.
It will compliment the DPH7500z processor nicely. THey are roughly the same size.
I also picked up 3 sets of 4-ch RCAs from KnuKonceptz. I contemplated building my own but when I saw the price of these was going to be cheaper I decided to save myself some work. I hope they don't introduce any noise into my system. One will go from the processor to the crossover. One will go from the crossover to the A5. and one will go from the crossover to the A8. Perfect!
On to other news, I also finished the subwoofer baffle build up. I'm starting to get excited about this. I can see the end! ...but I'm not looking forward to fiberglassing. It is my least favorite part, especially when I have to work inside the car.