The Speakercraft LL-One arrived in the mail today. I installed it this evening, and it seems to do exactly what it says on the tin. Switching from XM receiver to phone is almost instantaneous when the phone audio comes on, and I have it configured to switch back to XM when the phone is quiet for 5 seconds, which seems pretty reasonable.

I didn't have time to dial it in to make sure it stays on the phone audio when the music is softer, but I'll test it out some tomorrow and adjust this weekend if necessary. Audio levels between the XM receiver and the phone are pretty balanced with the phone volume pegged, and there doesn't seem to be any clipping.

I haven't hooked up the telephone mute line yet, but I plan on doing so once I figure out how to wire it properly.

All in all, I'm pretty pleased with the device. For $50, it's a pretty elegant solution to the "not enough inputs" problem.