I own a 2016 Civic Coupe EX-T, and I have a rear sub which I unhooked altogether and used
a "Line Output Converter" to send an RCA signal to an Aftermarket amplifier hooked up to
a pair of 12 inch subs in a pre-fab ported box.
With the bass and subwoofer completely turned down on the head-unit,
I still get magnificent bass and more clarity through the door speakers.
Unfortunately, the OEM amplifier has pre-determined sound processing
that makes the subwoofer low-pass at about 75 hz.
and the door speakers still gets a significant amount of bass which
causes a distortion at higher volumes.
The best option for completely independent user configuration is
to purchase something called a digital processing unit that accepts multiple high input signals
like the AudioControl DM-608 which has full customization of audio preferences.
There are cheaper products on the market, but I doubt will perform as well.
Sorry for digging up an old thread. I just wanted to clarify on additional options.
The sound-deadening materials will probably work, but I'm fairly certain my
coupe already has a ton of it in the doors which sound fantastic, already.
I've noticed a ton of rattling on the rear deck behind my back seats which I've seen in
other threads that people have had to do to reduce or eliminate rattle in the rear.