I've seen a lot of posts about getting sibelius to work under x64 versions with the X series, and wanted to post what I did to get it working.  It turns out that two preferences in sonar need to be unchecked, otherwise you will get an "unable to start in rewire mode" from sibelius when you open it.  so, to get it to work correctly, you need to make the following changes in Sonar's "Preferences/audio/playback and recording" section.  UNCHECK "share drivers with other programs" and UNCHECK "always open all devices".  
Sibelius should now start as a rewire device with no audio engine issues.