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