Using Mac as external audio amplifier, while routing its output to USB?
I found a solution: there's a free little OS X utility called LineIn (written by Rogue Amoeba Software). It just lets you route any audio coming in on the line in connector to any destination, including the built-in speakers.

So now I've got Garagaband routed to USB Audio out.. to the mixing panel, and then its output is piped back into the Mac via the Line input, the LineIn program, and finally to the built-in speakers.