Correct the E10 does not work with any source other than a computer by USB, and is not portable....
I guess the important question is are you looking for a desktop or portable solution?
If portable, there are lots of reasonably priced options out there. I am personally a fan of the Fiio E11 (the amp sounds better than the E10 to me, dirt cheap at $65, not including ipod line out dock cable), but I have only tried a limited number of options in this category. If you want to add a DAC there are two type to consider: Firstly there are the ipod direct USB connection compatible DACs, unfortunately all the options I am aware of are well north of your $200 budget; Secondly there are portable amps which also have a DAC for computer/source use. If your are at a desktop you could use a ipod digital dock (such as the Pure i20 ~$99) with these. In this category, you might want to investigate the Fiio E17 which also has low output impedance and comes in at ~$200 (I have not tried one personally so I can not vouch for it's performance).
[size=10pt]For desktop use... All of the units in your price range that I am aware of/ have personally tried have higher output impedances[/size][size=10pt]
. Perhaps someone more knowledgeable in that category could chime in.[/size]
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FYI here is a great article by Tyll at Inner Fidelity that touches on impedance mismatches in relation to the Beyerdynamic DT880 series (32,250and600ohm)[/size]
http://www.innerfidelity.com/content/comparison-beyerdynamic-dt-880-32-ohm-dt-880-250-ohm-and-dt-880-600-ohm-headphones
Hope that helps....
Brad