+1.
Mopar un-cut rear mat, no issues, no mess in the sub. I will add that I leave the factory carpet mat in place with the rubber one on top of it to give the sub a bit more breathing room.
Yes. My OE mat is under there as well. Bass is different than Tweeters and Mids which are directional and rely on an open line.
As mentioned, the Sub relies on the "coupling" to the tub.
I have A/B'd the Sub covered and uncovered. I won't lie. I can tell which is which but it is so subtle - not worth mentioning.
I'll add. My Dogs Do Not like the Sub. I couldn't figure out why they were whining a couple weeks ago and it hit me, "They can feel the floor moving" Doh! I turned the Bass down to zero and they were fine. Bad Daddy.