Add a subwoofer to the center channel?

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.     

@russ69 I suppose I could try that but turning these on their sides so the driver facing forward isn't going to fly aesthetically, at least for more than a day.

Surely someone has experience going from a smaller center to a larger full range center..  I would think that would be an equivalent.