You should at least have a decent DAC converter between the optical digital outputs of your Mac Pro and the analog inputs of your Genelecs. For example, an Apogee Mini-DAC
http://www.apogeedigital.com/products/mini-dac.php
Naturally your system would be compromised with the internal clock of MacPro but this would still be miles ahead of using the built-in analog line output of a computer.
A small interface could be better solution, overall, but your money is never wasted on Apogee so that's why this is what I bring up.