New Rel T5i. Anyone heard them?

Because the center channel, just like the front L/R and surround channels, is a full range channel, and it is independent from the LFE channel.  I like to hear the full frequency range of that channel.
The problem comes in particularly with deep male voice, a good example of which occurs with Russell Crow's voice in Mummy.  If you saw the movie at a theater, you know his voice in some scenes should be positively subterranean.  I don't think the LFE channel carries the lower frequencies of his voice.  And I have tried in vain to set crossovers on both subs and AV receivers to get that vocal quality in deep male voice.  The only way to get it is with a true full range center speaker.
I have seen a store demo of a HT setup in which they used the REL Ti series of subs to augment and extend the bass of the front L/R channels, and the improvement was great.  They employed a separate sub for the LFE channel. 

Unless you have a very unusual processor, when you set the crossover to high pass the mains/center/surround, all frequencies below the the high pass should be low passed to the subwoofer in addition to the .1. Running a center channel full range (unless it is enormous, which your’s is not; even Mark Seaton high passes his Catalyst mains for HT use, and they are huge) is only going to limit its output and increase distortion. You are also going to have phasing issues running your speakers full range with subs overlapping.

Most likely what you have heard at the demo was the subwoofer level was set much higher than the mains, given a big boost even into the 100-120hz range. It is done all the time to make demos impressive. While not technically accurate, many (possibly most) home theater enthusiast run their sub(s) hot. I suggest playing with that and/or adding addition subs to get the sound, not running your center and mains full range and over driving them.  You may want to pop over to AVS Forums and open a discussion there, as they are home theater focused.  

“While not technically accurate, many (possibly most) home theater enthusiast run their sub(s) hot.“

True for novices.  Absolutely DO NOT agree for any serious enthusiast.    If you have a properly calibrated and room corrected system, turning the subs up even 3db sounds so obviously bad.  Someone who wants to HEAR the subs working and have them stand out is not someone who I’d ever trust for audio advice in any way.  And that’s a huge problem with AVS forum.

I have found the AVS guys to be pretty good and have a lot of data and information on setup for home theater.