The 2910 is an upgraded 2200. The 3910 has better DAC's, better build quality. I have not a/b'ed them. Can not comment on if the 3910 is worth 2x the price.
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Some reviewers have, and they comment that the 3910 is NOT worth the extra price.
I now own the 2910 so before I purchased it I was doing some research on the model line and the result was getting the 2910 for $585.00 AT A LOCAL FULL SERVICE DEALER.
The 3910 is not woth the huge difference in the price.