Someone's probably going to tell me that this is a bad idea, but...

[GMichael]

I used the Infinity 1060W Reference Series 10" 4-ohm Subwoofer from here http://www.audio-warehouse.com/web/mdl/1060w/detail.asp
It worked great for me in the trunk of my 98 Civic.
It's only 91db though.

[Ged]

Thanks, GMichael. If I decide to go bigger, then I'll almost certainly go with an outboard amp. All options are still on the table while I consider what I'm able to do.

Pioneer tech support says the continuous power output of the rear channels "combined" is 40w.