will a power amp NAD 216 add any improvement

thanks alot, this is a great help.

The power rating doesn't matter much, it's the gain of the power amp that matters most. The power rating gives an indication of the maximum sound pressure you can achieve, where in this case the 216 will be able to achieve a higher max. sound level than the other 3 channels.
I'm not sure the 216 has an input level, but if it has, you'll have to adjust it with a test tone so the sound level matches centre and rear. If the 216 hasn't got an adjustable input, you have to change the level of centre and rear channels so they will match the front. This can probably be done through the menu of the T753, check the manual for that.
Either way, it would be handy to have a device to measure sound pressure and maybe a good test DVD. They won't cost the earth, but will be of great help setting up your system.
Good luck!

Thanks for your answer!
Is it going to make a difference the fact that the power amp has a different power rating than the rest of the channels? The power amp has 125W and the T753 has 70W. Probably I will have to adjust the input level on the power amp.

Yep, that will be a big improvement. The good thing is that it will more or less match the sound of your NAD T753, but will have much more power and control, which is just what the Linn's like. Also your normal stereo sound, provided you use the 216 as power amp and the 753 just as a pre, will improve. Good luck!