Generally speaking, the rule of thumb to follow is that for a given headphone, a headphone amp with an output impedance of roughly 1/8 (or less, I believe) of the headphone's impedance will provide the best match. So for your 25 ohm D7000, it would be best if the headphone amp you chose had an output impedance of roughly 3 ohms or less.
I believe the NuForce Icon, as well as the FiiO E9, have an output impedance of 10 ohms. The SPL Auditor has an output impedance of 9 ohms. While any of these amps would probably be just fine with the D7000, if one is to believe the science, an amp with a lower output impedance would be preferable.
For example, I think I read somewhere that the Schiit Asgard has an output impedance of 2 ohms. The HeadAmp GS-1 has an output impedance of less than 1 ohm. These would probably both be better matches for the D7000, at least in theory anyway.
Bottom line: any of the three amps you mentioned would probably be just fine with the D7000. If you're going to restrict your purchase to items you can buy in a shop (i.e. not online), then any of those three would be good. I haven't personally heard any of them, but I have read very good things about the SPL amps.
Another one maybe worth considering, though I know very little about it:
http://www.head-fi.org/t/514107/review-casea-lyra-mk-ii-digital-solid-state-headphone-amp-dac