Add a subwoofer to the center channel?

This really sounds like a poor system integration problem you should not need a new sub to fix. In my set up I use 1 subwoofer and there’s just no difference. The center channel does not sound tinny, and effect sounds don’t sound like they are hollow in the center.

I’m not familiar with Marantz but make sure the center bass is set to go to your main or subwoofers.

It also sounds like you really really want another sub.  If that's the case, go right ahead. :) 

Ha well I have two REL t5i's and a KEF r400b hanging around that I like to put in different places.

I had the RELs on a soundbar and it sounded SO good for a bedroom or send room system.

The REL T5i is -6 db at 32 Hz. That is just barely a sub in my book but you can still improve what you are hearing. I would place them in a forward firing stereo configuration. Your crossover setting will be fairly high and you will be able to locate the subs so having them in a stereo configuration will help with imaging. Try that first, I bet it will clean things up.  

@russ69 Yeah RELs aren't known to be rated down that low I agree.. These are very small subs so they're not output beasts anyway. But adding them to a soundbar, I can almost promise you you won't miss the sub 20hz lack of performance.

Anyway, I guess my center/sub idea is shot.  Best to let the SVS subs handle the center bass.  

I thought about using two center speakers but after reading more about it, that sounds like a terrible idea too.

Nothing "wrong" with what I've got now.. I LOVE the sound, I just figured filling out the center breadth would make the overall experience more evoking.  

"... just figured filling out the center breadth would make the overall experience more evoking..."

I gave you a clue to do exactly that. Give it a try. Zero cost, except some time and sweat labor.