I'm having the same issue. I'm running SD 18.104.22.168 on a VIA PX-10000.
I was having a heck of a time with the HD Audio driver- now that works fine, but I can'd get the GPS and XM-PCR to run at the same time without throwing an error from the ser2pl driver.
I've ordered a MR-350P Bulkhead with a serial interface to replace the Pharos-360 GPS antenna.
Other than that... I'm not impressed with the XM over PC sound. My MP3s sound excellent- XM... not so much. Wish there was a way to run the XM output through the SD audio enhancer.