I have the Quad and it's been driving me crazy with Sonar Xe for quite a while. I've had many lengthy discussions with Roland on the matter without a definitive answer as to why Sonar doesn't see the Quad when it stops working. Sonar typically 'hangs' when it stops working and stays on the system - in Task Manager - for up to 5 approx minutes. I can then restart it but I get the message from Sonar that there's no audio device on the system, even though Windows can still play sounds. I have to un-plug the Quad, test it and then Sonar will see it when I restart it.
I've had to experiment with different buffer settings in the Quad and change the amount of buffers in 'audio' in Sonar to find the 'sweet spot' where it all works well. Too low of a setting in the Quad and I've had no sound at all. Putting it up to 7 or 8, has the sound back but of course latency is evident.
Try experimenting with different buffer settings in the Quad. My sample rate is 48k, buffer size is set at 8, I have the 'low latency' unchecked. Sonar buffers something like 512 or double that.
I'm haven't had this issue - yet - in platinum.