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Really? The reason I got the E-17 was because most reviews on them were positive.
Right. There is nothing wrong with the E17. It's definitely a good unit. However, depending on what headphones someone uses, how loud they listen to music, and the sound coming out of a laptop or computer, it may offer a very minimal upgrade in SQ (which may not even be noticeable) that may or may not be worth the cost to some people.
I don't know anything about surface area and DACs, but the Xonar STX is a better sounding DAC/headphone amp than the E17. That's generally very much agreed up on Head-Fi. Having owned both myself, I would say it's somewhat easy to hear a difference. Xonar DX/D1 would not be for most headphone use since it does not have a headphone amp.