Quick update. The store is giving me my money back and I am going to keep playing my Martin Magna for a while. I only moved away from the Martin on the advice of my former teacher who thought its medium bore would hold me back. Now that I've been playing it again for a few weeks, I've never sounded better (read: smoother!) and I'm loving it. My new teacher didn't realise I had it and when I showed up for my lesson last night he was wondering why I'd bothered with the Jupiter in the first place. He strongly suggested I just play the Martin and enjoy it. Think I will!