Don't know why I never thought of doing it before, but yesterday I hooked up a Shure SM58 and my Taylor acoustic/electric to my Mackie ProFX12 mixer, then sent the output to the CD/MP3 input on the back of the Spider.
Voila! - Instant PA System!
I did find out, though, that if I only fed 1 side of the left/right mixer output to the stereo input that the volume was not very loud, but when I sent a full stereo signal (I used the headphone output because I only had an adapter to do it that way), the volume was huge - and clear!
Most likely I would never use the amp this way, but it's a good option as I don't have any powered PA speakers of my own to use, and it would make an ideal monitor, too.