Subwoofer with center channel

Sounds like you’ve got your priorities straight in terms of more highly valuing 2-channel. 😜  If possible I’d have all four subs working in a swarm array for stereo assuming you could have both LFE and L/R line-level inputs connected to the SVS sub simultaneously.  That’d be your biggest benefit in employing your NHT sub IMHO.  FWIW. 

@soix A speaker level connection with the SVS and NHT would be difficult.  I'm currently sending the LFE channel to the SVS via a REL wireless transmitter.  And, I don't really have a great spot for the NHT.  It would be interesting to try four subs, though.

It's a shame to not use the NHT.  It's a nice subwoofer.  I could swap it for the SVS, but the SVS is probably better for movies.  The NHT has two drivers in a sealed enclosure with an outboard controller.  I had NHT replace the capacitors and they also replaced the tracking voltage controls a couple of years ago.  They were very helpful and reasonable about costs.  I think I paid $150 for the repair..  

I run a subwoofer with my CC. Mid-bass or bass bin is a better description. I have thought about replacing my old Infinity Prelude Compositions; Mains - Model P-FR (2 ea.), and Center - P-CC, but they fit perfect in my small theater and still sound great and the foam surrounds are still as new.

The one deficiency was the center channel would only go down to 150Hz when running Audyssey on my Marantz AV7705. I had an unused Rane AC 22B crossover and an AuraSound NS10-513-4A in 1 cubic ft sealed enclosure I had built along with a spare amplifier channel and added it under my center channel to help fill the gap. I ran Audyssey with the bass bin added and the CC is now set at 80Hz and dialog and CC sound seems fuller and more natural. I have a TC Sounds LMS Ultra 5400 18" and 2 ea. LMS-R 15’s for actual subwoofer duty.

I think we may be confusing/conflating two different issues in this thread. I read the original question as applying to a center speaker with an integral subwoofer, not a separate sub altogether. I have four different AV systems, and one of those uses a Definitive Technology CLR 2003 center speaker, which has a built-in subwoofer. Set correctly, it does not induce any  boominess or chestiness into male vocals. It does make a big difference on things like explosions/gunfire, giving much greater impact in the center channel. Of course, I also run subwoofers (at least two) in each of the AV systems.

@discnik That is the how I interpreted it. A subwoofer, used as a mid-bass in my example, functions just like a speaker with a built-in subwoofer, it just uses an external active crossover. If the OP wants to continue using the center channel they like except for bass extension, adding a bass bin via an external crossover that only addresses the center channel signal is a potential solution and the one I use. I use a high pass of 80 Hz and I'm not sure but around a 150 or 160 Hz low pass between the bass bin and center channel. This is right in the sweet spot for the AuraSound NS10-513-4A in 1 cubic ft sealed enclosure.