Using a Rel Sub with a dedicated center channel

Yes, subwoofers are great, no you don't need more. :)

So long as your center is set to small ( instead of full range) your HT processor is already sending the bass to the subs in addition to the LFE.  You already have the benefits.

Personally, I go the other way. My center sits on top of my sub, and my front 3 speakers are set to small.  Works great. Before my sub was under the left speaker.  Sounded exactly the same, wonderful. :)

I have seen only a very few movies where deep bass effects are mixed into the center channel.  Most movies will mix bass into left/right or the .1 LFE channel.  A subwoofer attached to the center channel signal won't really make any difference in most scenarios. I think the more important choice in a center channel is to choose a speaker that can play low enough (like 45-50 Hz).  Then run the center channel full range as LARGE or cross it over very low (such as 40 hz).  I have found that it's important for the center channel speaker to reproduce the lower octave.  A LOT of detail occurs here in male voices and running center as LARGE or 40hz crossover will give you a better cohesion on the low male vocals.  It's actually much better than shunting that to the subs.

Could the sub potentially fill in the 60-120 hz range for the center channel though? The vocal clarity is so good on my center I would fear trying something else. I've tried a few centers in the $1200-1600 range and I definitely like this one the best.

I would probably get a Rel T9/i or S3/sho, nothing crazy

Could the sub potentially fill in the 60-120 hz range for the center channel though?

You mean like your two existing subs are now??

I'm not sure you understand how this works in HT. The HT processor is already sending this to your existing subs. You aren't losing anything.