Ironically, the only rattle that has given me trouble is the stupid 3rd brake light housing. Something I never took apart originally.
The interior is actually quite easy to take out. The clips are somewhat disposable so I have a 100 pack from Amazon and replace them if they're loose or bent.
Getting the woofers out of the doors is good for rattles, since the door cards are the biggest offender.
With that said, I feel like the underseat midbass isn't the best approach, since a good portion of the energy goes directly into the cushion. At high volumes you get a butt massage instead of nice midbass.
My next project will be to move the 8's BACK into the doors.