Suggestion
I use the TC-Helicon Voice Touch. It has separate headphone and speaker outputs so feedback has never been a problem.
I'm sort of fortunate because there is no reason that it should work at all. It has a separate mic xlr, an aux stereo input for my keyboard and a lot of other ins and outs.
I had a problem once. I've used the iPad 2, the 3, the 4 and now the iPad Air with 128 gig. There was the feedback you have described. It was for a short period and it went away?
I forgot what I was doing at the time but the mic on the iPad was open, why, I don't know.
If anyone else is having a problem it may be associated with the new 64 bit GarageBand, I really doubt it but I also checked the crosstalk switch, it was in the factory set position to off.
I still don't know where it came from but recently it came back. I adjusted the headphone and speaker knobs to the off position without a mic plugged in and it didn't matter, the feedback was still there. That narrows it down because my keyboard is a direct input. It has to be an open on board iPad mic.
I haven't had time to mess with it but it will present a major problem.
I'm sorry for this non answer post but sometimes I read someone else's and it ticks something and I usually find the problem.
I am using Garageband 09' (version 5.1) with a M-Audio Firewire Solo. Whe I connect the microphone with an XLR cable, it works fine. However, when I connect the acoustic guitar (I'm using the Dean Markey transducer for my pickup), it's not very loud, and when I turn the volume up to get any decent volume, I am always getting some feedback. I've tried different patch cables but I'm still getting feedback. This is so frustrating. When I play with the recording level in Garageband, it doesn't change. It changes when I adjust the level on the Firewire Solo.
Anyone have suggestions as to how to fix it? The Firewire Solo is old....it's from 2006. So I wonder if that's the problem. But I wonder if I can solve this without needing to buy something new.