I took the car to a guy that does electronic surveillance installations on fleet vehicles, stereo installs, etc. He put in a rap CD and had me touch the cone on the sub. Sure enough, they were vibrating! He also pointed out that all of the speakers are controlled through only two of the wiring harness's jacks so I am using only the center jacks!

My back must be particularly insensitive as I do not feel any vibration coming through my seats from the subs.

Sorry to waste everyones time!