[aggrex]

I'm thinking about pulling the sub out and replacing it with something else. Has anyone else tried putting a 3way or 4way speaker in its place?

The sub and sub space is much larger than the typical door speaker. Since you’re planning on running new speaker wires mount a pair of speaker pods in the back. The sub would still remain functional.

 

[aggrex]

That's not a bad idea! If I wanted to avoid running wire, could I use the sub's wiring for a non-sub speaker?

Not an audio specialist but I think the sub wiring is a separate line for the sub only. Hang tight as there are some Piloteer audio aficionados that may have input (pun intended) on the subject.