[SoNic67]

I wanted to add a subwoofer to the existing back door location

I admire the work that you put into this. To me there is plenty of low end even with the factory subwoofer (connected to the OE JBL/Harman Kardon amplifier).

I also hate if anything rattles when listening to music, I hear that daily on Hondas

What I didn't like on the OE system was a pronounced "hump" at 9kHz (that some people used to mp3 from phone trough in-ear headphones like) and lack of real top end (14kHz-20kHz).
I measured this audio curve with a sound generator app, and the microphone of my phone. That's why I have replaced the front door factory "tweeters" with 3.5" JBL GTO329 coaxial speakers (premium line, 3 ohm). Yes, those are a bit larger than the panel cutout, but I left it as is. They filled nicely the mid-top end.

I have my rear speakers replaced with kicker CX series speakers

Do you have the factory amp? If so... do those speakers have 3 ohm coils?
Personally I am looking at the JBL Club 620F series (3 ohm/55W/92dB) or Infinity Reference REF-6532ex series (3ohm/55W/93dB).

 

[SoNic67]

Ah, yeah, if you didn't have a sub to start with, any sub will make a difference.

 

[SoNic67]

The RF is wired in series to get 4-ohm since the mono amp isn't rated for anything lower than 2-ohm.

Wonder if one single coil is rated 100% of the power? In that case you can use it alone, at 2 ohm, and thus getting most of the power from that sub.

 

[SoNic67]

In my experience I always "broke" a speaker by mechanically hitting the extremes and misaligning / bending the coil. I never melted a coil wire like a fuse... but I don't know if that's always true.