help on center speaker

"It does not make sense to buy good/full range speakers for home theater and then to send all the low frequency to the sub."

When one uses full-range speakers with good bass response, one simply doesn't 'send all the low frequency to the sub', one leaves the low frequencies that are indeed present in the other 5 channels in those channels. The result is more directionality to the overall sound AND a less-loaded LFE woofer.

"I believe a good sub with cheap speakers will do the job as well."

Small speakers and 1 sub probably will never sound as good as 5 full-range speakers and a subwoofer.
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Ok, let me make sure I understand this correctly: if you setup the speakers to full range (large speaker in the NAD's setup), the low frequency will not be cut for those speakers - for example my Linn Index's low range I believe is from 60 Hz. The rest of my speakers have a low freq range of 80 Hz.
In this case do I want to set the sub crossover to 60 Hz and front speakers to LARGE or I still have to set the sub to 80 Hz with front speakers to LARGE?
My intuition tells me that the second setting is the correct one.
Thanks a lot for the help!

If you can, use the same speaker that you use for your mains for the center channel. Actually use the same full range speakers all around.With large speakers this may mean building a new baffle to rearrange the drivers. It may mean building a different shaped cabinet to house the drivers, just keep the volume the same. Build a special cabinet to hold it over the TV, or put it behind a perforated screen. Be creative and come up with a way to use the same speaker. You will be glad that you did. The center speaker is very important, I wasn't satisfied untill I matched mine up.

Full range speakers arn't a waste. They allow you to set the crossover frequency to the sub much lower the 80Hz. Mine are set between 35 and 40Hz.

no, I would set the NAD crossover at 80hz (all channels) and all speakers small. Turn you sub crossover all the way up or disable it. Remember LFE is 120Hz and lower, if your subs crossover is in the way you will be missing information. A large speaker never sends anything to the sub, only small below the crossover setting...

I will keep it at 80 hz and small speaker setting, until I upgrade my front speakers.