I have read a lot of sub install threads, but I haven't seen anyone specifically address removing the free-air sub from the dashboard in the sedan after the new sub is installed.

Are there any pros or cons to leaving it in or removing it completely?

I was planning on removing it, as it is one less thing to rattle, but didn't know if there are any specific motives to doing it one way or the other.

 

Pro: Removing it can open a port between the trunk and cabin.
Pro: Removing it will prevent it from being turned into a passive emitter out of phase from the rest of the sound from the pressure in the trunk from the real subwoofer.

Con: Removing it reduces weight from the rear deck, potentially increasing its propensity to vibrate, so you'll want to deaden the panel if you remove the mass.