Add a subwoofer to the center channel?

I think the end to your question comes when you ask yourself, exactly how are you going to configure and set up a center only sub?  Work through that first and get back to us. :-)

The issue with center channel speakers tends to be resonance from where they have to be placed.  Make sure you are taking advantage of room correction while it is engaged.  If this is done correctly then you should not be able to hear any difference with the sound quality as effects move across the front.

Setting the center to "large" will only distress it. 

@erik_squires 

I was going to use a REL t5i and use the Neutrix (ms i know) connector.. So set the center to Large and use connect the sub directly to the speaker posts as you'd normally connect a REL.  This would create one "Large" center.

REL does this in their 3d cinema demo room. They put subs on the front, rear, center and have a dedicated LFE.  They were discussing how having a "full sized" sounding center adds alot which kinda makes sense.

Right now, I guess my centers low freqs are being handled by my big SVS's which doesn't seem ideal for them to handle that when they should be dealing with big LFE stuff.

Idk, again, I just figured this would come much closer to having a matching sized center.  

Also, I have not used room eq yet because I don't like Audyssey (marantz av7705mkii).  The vocals do sound like they're coming from a smaller less robust speaker.  

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.     

@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.