If you are just after something that will power your HD600s, through your Ipod, I would have thought that the E11k would have enough power to do that.
This will not even be close though to what your headphones are actually capable of sounding like. If you like the size of your Ipod, then by the time the E11k is added, it will be only a tiny bit smaller than a decent music player such as the FiiO X1. I would have thought that you would be able to get much better sound out of that.
I have the X3 which is better still and am using headphones that won't be quite as good as your HD600s.
It will work fine but you will be able to get much better sound form the cheap X1.
I'm not quite sure what you mean though when you say you love the size of your ipod. Are you planning on having it strapped to the E11k? If you do then I would have thought it would be less practical that getting the X1. If you really like your ipod then stick with it. I know the X1 is more expensive than the E11k but I just don't feel like the the ipod and the E11k will get the most out of your headphones. If you have listened to your HD600s through a very decent amplifier somewhere else, then the ipod and E11k together won't compare.
The E11k is designed more for connecting to the line out of players such as the X1 ore X3 or similar. Iphones too maybe. But will headphones as good as your HD600s, I am quite sure you will get much better sound with something else.
Hope I have helped a little.