I think the D10 is probably the best choice based on what you've told us, because you can use optical with iRiver and Macbook at 24/96. And it's portable for using the iRiver with your ESW10. You can even charge the D10 via USB while you use the optical input, for leaving it hooked up to the Macbook all the time. The D10 sounds nice with HD600 but it it could use more power for those, however you could feed the D10 into WA6 or Protector for the HD600 (I've used HD600 with both, and am burning in a Protector right now).
Of those you are considering, the only other one I have heard is the Pico DAC, and it's very good, and on a similar level to the D10 optical at 24/96 - maybe slightly less detailed when using native 24/96 music, but no difference with lossless 16/44 CD rips. One problem is it's computer only, and not compatible with iRiver.
I think the DACport for $399 is another to consider because the 24/96 native USB DAC is just as detailed as the D10 but the headphone out sounds better. Like the Pico it is computer only. The DACport's headphone out works extremely well as a preamp feeding my Protector, which is what I am using as my source to burn-in my Protector.
All three of these are close enough in performance that you could likely flip a coin and be quite happy. For USB I think the DACport is best, followed very closely by the Pico and then D10 very closely behind that. For optical the D10 as a DAC only can compete with the DACport's 24/96 USB, but the headphone amp is not as good, even with upgraded opamps in the D10. For portable, the D10 is the only one.
