After some more fiddling around tonight on GarageBand with my guitar hooked up I've figured out the cause of the problem. As it turns out there is next-to-no issue with my sound board, powered subwoofer, recording input, etc. The buzzing noise from the guitar is... strangely enough... coming FROM the guitar!
I did a few more tests, unplugging the powered subwoofer to see if that was it and I still got plenty of feedback when listening to the guitar plugged in. Then I started to wonder, my acoustic-electric takes a single 9V battery but the cage where the battery sits has been a little loose lately. The connections in there are a little wonky too and I wondered if something might be causing the feedback.
I finally had the novel idea to connect the guitar to my amp, completely removing the recording and computers from the equation. Sure enough, guitar plugged straight into the amp, there is the same grounding buzzing hum noise!! Found it! The guitar amp is grounded too so that's even better than the sound board I was plugged into which isn't grounded. Interesting. So the problem must lie in some grounding issue inside the guitar's pickup itself. I betcha I can take this into Guitar Center and have them take a look and fix it. Awesome.
When I plugged my electric guitar into the same amp I tested to see if it had the same issue and it very clearly doesn't. I remember that I had some issues with the electrical connection on that a while back and I decided to have a go at fixing it on my own with my new mini screwdriver tools. That worked out great, just tightened up the connections and it's working nicely again. So that's good news, I don't need to take my electric in anymore, but I do need to take in the acoustic-electric.
After fiddling for a while I wanted to get back to fun stuff so I plugged the electric into the sound board and everything works nicely, no gross buzzing feedback and it seems to be working nice and cleanly. The other cool thing is I tried out the "Electric Guitar" instrument on GarageBand... holy cow!!!!!!!!!!! There are some amazing effects and I love the images of the stomp boxes and SO COOL that you can fiddle with the knobs and effects settings. The amps are REALLY friggin' cool too and it's amazing all the presets they have. There was one called Woodstock Haze or something that sounds very Jimi Hendrix. It is really badass. Definitely makes me want to play some electric more!
The acoustic effects on GarageBand are definitely nice (I am a sucker for chorus and some nice auditorium-style reverb) but the electric ones take it up a HUGE notch. I am so glad I have this software, it's really amazing. Definitely encouraging me to play some more guitar. Fun!
Anyway it's been a pretty crazy day so it was nice to change gears a bit and play some fun guitar and work with GarageBand a bit. Also nice to finally determine that my computer and sound board aren't causing the crazy feedback problem from the guitar, I'll just have to take it in and ask them to fix the pickup.