Yeah, that looks doable, though I'm wondering if the lack of amplification might be a problem. The XM radio unit audio signal comes out really hot, whereas the headphone jack on my phone comes out pretty quiet. If I try to balance them without any sort of preamp, I'll end up sending a really weak signal into the empeg's AUX in. Maybe I can correct for that with whatever the Hijack setting is for that, but it seems like there might be less noise if I found something that would amplify the signal before going into the empeg.

This looks like it might be an attractive mini mixer option, with an integrated preamp. If I don't end up needing the amplification, I just won't bother to wire the 12V DC in, but it'd be nice to have the option. Looks like it can be had for around $50.

Or, if I wanted to try to go the switching instead of mixing route, I could pony up about $50 more for this guy which will do either automatic switching based on silence detection or manual switching with a remote pushbutton. It's a bit more of a hassle to install, and I don't like the non-configurable 10-12 second delay when switching from the higher priority source, but it seems like a pretty elegant, if expensive solution to the problem.