When you start Sonar, it sends out MIDI messages to switch off local control on your synths. This is what is causing your X Station to 'lock up', if you go into the synth global menu on your X Station (Press the 'Global' button twice when you are in synth mode) you can see that the Synth Local control option has been set to 'off', change this to 'On' and you will be able to control the X Station using its controls.
It is possible to stop Sonar from doing this by editing the initialization file. This is a file called "TTSSEQ.ini" and you will find it in the Sonar program folder in program files.
Add the following text to the file (using windows notepad).
This command will then stop Sonar from sending the MIDI message to disable local control on start up and hence the X-Station will respond to its controls as normal.