I had a good look with my Cakewalk Pro Audio 8 demo. After selecting the soft synth as the output port I assigned the channels to the Yamaha XG instruments from the track properties screen. And then I had a look at the bank listings and here all the instruments were listed for aprroximately 550 voices. It's not the full 676 but I'm not sure what that refers to - theres a full XG listing here as well
http://www.yamaha.co.uk/xg/reading/index.html

in the specifications pdf and that has a similar number of voices.