Finally received all my audio parts! They'll serve as garage decor until finals are over next week.
Here's everything I ended up with, after much budget shuffling and research:
Processor: AudioControl LC7i for cleaning up the muddy factory audio and adding a dedicated sub channel.
Amplifier: Rockford Fosgate R600X5 5-channel amp with 50Wx4 and 200W sub channel.
Front components: JBL GTO638 3-way components
Subwoofer: Pioneer TS-W260D4 I chose this sub so I could run the dual voice coils in parallel, gaining a bit of efficiency. The box volume recommendations fall in line with the general dimensions of my custom sub box, see next item.
Sub Box: This will be my own custom design, but styled after the JL Stealthbox. I plan to integrate it as neatly as possible into the rear hatch area, minimizing intrusion into the cargo area. I have a couple enhancements I might design in as well, such as a cargo light and a storage cubby for my first aid kit.
Misc: I got a 4-gauge power wiring kit for the amplifier, RCA patch cables for the processor to amp, and 16-gauge speaker wire. I got male and female Subaru stereo harnesses from Subaru radio and radio wiring harness information and will be soldering a custom plug-and play harness, just as I did for my WRX. The harness will intercept the factory audio outputs, process and amplify them, then feed them back into the factory wiring. I'll have quite the wire bundle to feed through, but at least I won't have to run new wire through the doors. I also got Legacy-specific component adaptors for installing the front components.
Here's the rough layout for the processor and amplifier. Both fit neatly in the factory storage box above the spare tire. Subaru conveniently built natural cooling ducts through this space as well. How thoughtful. I even have space left over for my tow cable, jump cables, tie downs, and other emergency supplies.