Using a Rel Sub with a dedicated center channel

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.

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.

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.

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.

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.