I'm also curious about this question. I just purchased a set of 350mc's and am considering the upgrade to the x series.
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This does not exactly answer your question but I upgraded my fpb 200c to a 300cx and it became more open (less dark sounding) and more refined detail. My dealer raved about the difference I would hear. As it turned out it was a good difference, but not huge or dramatic. I am glad that I upgraded however. I would do it again even if there had not been a power increase.