Ear Abuser

Where I come from, they have been amplifying accordians since the early 60's. Every wedding reception had a guy playing an accordian through some kind of amp.

You might also try and find the oldest music store in town and ask around for someone who does accordian repairs. They may have some ideas as to a better mic/pick-up arrangement.

Have you tried cutting the offending feedback frequency(s) with an equalizer? You could try a 31 band graphic or a parametric equalizer.

There is also a Baggs acoustic guitar preamp that has a tunable notch filter that might help.

Then again, this could a sign that you shouldn't try to amplify this particular instrument.......