Center Speaker Suggestions??

I have a 5.1 system with all NHT speakers driven by a Denon receiver. The center channel (which does the yeoman's work when playing Blu-Ray,) is unintelligible. When using my Denon wireless headphones, the sound is glorious - very easy to understand dialogue... so I'm thinking something is not right with my NHT midrange speaker. Does anyone have a suggestion?
Such problems are common and are usually due to poor positioning/setup of the speakers, room acoustics and/or wiring errors.

Suggestion: Give us the details.

Kal what details are you looking for. The various speakers are connected with 2 conductor speaker cable to their appropriate outputs to the Denon being mindful of keeping the pos and neg in their appropriate outputs and inputs on the speakers themselves. You may have something since I get better intelligibility when just watching TV. Inputs are digital from the Panasonic bluray to the appropriate input of the Denon. The TV signal goes from the Panasonic or cable box to the appropriate inputs on the TV via HDMI...Great picture. There are no HDMI inputs on the Denon ( a few years old ). There is a microphone that balances out the speakers ....a program that is automatically done. What else??
Confirm all wiring connections, just in case of a silly mistake or bad cable. Check set up and levels in menu of Denon. Swab out center channel speaker with any other speaker just too make certain your NHT center is indeed working properly. Good luck.
Where are the speakers? Where do you sit in relation to them? Describe your room and major furnishings.
Might want to try different speaker size small instead of large.
Or check Dolby volume if on shut it off.
A receiver that doesn't have HDMI doesn't have Dolby Volume. I would try redoing your setup with the center speaker removed. I had some NHT speakers (Classic Three) a while ago, and the L/R were significantly better performers than the center. Depending on your room and seating position you may find that a phantom center is better than an inferior center.