I have a very nice electric acoustic guitar which has its own pre-amp - but whenever i actually plug it into an amp, if I stop muting the strings with my hand, the amp gives me masses of feedback. Any ideas what could be causing this?
Most semis and acoustics do that

try facing the amp the opposite way and sit/stand as far away from the amp as possible
The Official "Rockin' Roadie"

-Lakland Skyline 44-02
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^ Those would help. Often, guitarists put something over their soundhole in live situations. You could try that also.
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