Using a Rel Sub with a dedicated center channel

Anyone using a Rel subwoofer with their dedicated center channel in a home theater? Rel has a video on their youtube page from a couple years ago on how to do this and talk about the benefits a little. Someone in the comments makes the case that any channel in a HT can benefit from a subwoofer being added but it does have me intrigued. So much of the sound comes for a HT comes from the center channel it makes me think it might benefit. I am already running 2 SVS SB4000 in my setup. The Yamaha CX-A5200 I am using sets my B&W HTM 1 center channel at 120 hz for crossover and the Vandersteen 2ce sigs that are my are RL channels to 80hz. I actually really like the sound of everything right now but think maybe this can take it to the next level?
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.
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.
The B&W HTM 1 should have no problem going down to 50-60 hz.  I would set the center to LARGE in your situation and just live with that.  It's gong to be much better for male vocal and other things.
I just looked at the specs, after @auxinput commented.

I’m not sure large is the right answer, but 120 Hz seems like a very high cut off. 80 Hz is the THX standard, and a better compromise.
The problem with setting them to large is potentially more distortion, so my recommendation might be to try these two options. Try setting to 80, and then to large.

Remember that in either case, your HT processor should send the remaining bass to the other two subs. 
I’ll try setting it to 80 hz first and see what happens. I have been planning on getting a better amplifier to power my center channel. I’ve got a Marantz mm8807 for the center and surrounds for now. I’m sure it’s plenty good for the surrounds but B&W usually like lots of power. The Yamaha sets the freq to 120 hz when I do it’s ypao corrections. I was worried about setting it lower than the 120 as I’ve read you generally shouldn’t go lower?
Yeah, 120 hz is very high for a sub crossover. Some subs may actually cause beaming or a "too strong" type of sound in the midbass at this crossover. They are also not as crisp/clean in the midbass at these frequencies. I have a HTM2 D3 center that I run LARGE and it's been fine except for 1 or 2 movies that peaked the woofers at extremely loud volume.  Those are the 1 or 2 examples where the movie soundtrack will actually output bass effects into the center.  Otherwise, LARGE is just fine.  But if you have concerns, I would set the center crossover at 40 hz like I suggested earlier.
I played around with the settings on the Yamaha a bit, I think I'm also searching for something that might not be there - I really like how everything sounds as is too. Seems like a subwoofer may be more of a waste than a benefit. But as always, we keep searching for whats next to upgrade. 
The lower in F you go, the more power the receiver has to put out. That is, the power shifts from the subwoofer to the receiver, so that's a little bit of a concern, also, the more bass a small speaker plays, the more distortion.

Without seeing what the receiver is doing precisely, I suspect that 80 to 120 Hz is in a good range.  In some cases you may be limited to hte smallest speaker defining the crossover.  For instance, if you have really small surrounds, which MUST be set to 120, the receiver may only be able to use 1 crossover setting and will choose 120 for all speakers, even if your center is bigger.
He's using a Yamaha CX-A5200, which is actually a processor. 

chief - what amp are you using? 

If you're looking for a next step, maybe try to look at matching your three front speakers.  That would make the sound much more cohesive.  There are definite problems when you have mis-matched speakers for the front stage.  The surrounds are not as critical, as long as they are close in sonic signature.
I’m using a Marantz mm8807 to power my center, surrounds and height speakers. Could it be there isn’t enough juice in the amplifier? 
The Marantz should be plenty powerful.  The Yamaha receiver could be setting a high 120hz crossover point because the center may not be producing as much bass because of room nodes and bass node cancellation.  It's not because the center won't do it.  It is likely your room that is causing problems.  I would still set the crossover lower, such as 80hz or 40hz.  The room will have the same bass nodes regardless of where the bass is coming from.