Just an update on status:
I had some older Alpine SPS 170a coaxial speakers in there and hooked them up to the new Aura amp and the Kenwood Excelon x395 head unit. This is the first time I'd ever run an amp, and it is amazing. For now these speakers are fine with the new amp.
Component Speakers I am keeping an eye on for used or good price include
Infinity Reference 6.5
Polk DB 6500 6.5
Boston Acoustic SL60
Phoenix Gold RSD
Oxygen Audio Air
The Kenwood wOOx is running ok. It adds some fill but is just a stopgap solution. My current focus is on a 8-10" subwoofer to install either above the spare tire (do a search for subaru spare tire subwoofer and you'll find pictures of an interesting box that is combined with a large PVC pipe or sonotube and fits above the spare tire). Another location is using the right side smuggler's hatch and extending into the car some.
Small volume, shallow mount subwoofer's I'm looking into include:
Tang Bang W6-1130IF
Tang Band 8" shallow
Alpine Type R
Peerless XLS 10"
Elemental design 6.5, or 8" (out of business)