actually, I couldn't get the loopback test to function from my SX host either ...
*However*, I did get SysLink talking through XTC. The key for me was adding a MIDI track to the SX project window with monitoring enabled and the SysLink MIDI channel selected as the output port.
It took lots of trian and error to figure that out. Hope it helps. Yes, unfortunately there are some quirks in running SysLink, but it can work, and when it does, it's really pretty frickin' cool.