With the HD600 the better the amp the better they will sound. So... keep the E10 and get an o2. E17 is a portable DAC/AMP but i can't see why you would need something portable to use with the HD600
The HD600 will be fine with the E10, so take your time to listen from the e10 and burn the headphones, and if the volume is low or you are willing to pay for something better, then i guess the O2 is a good amp.
Is it really true that a "better" amp will improve the way a headphone performs? From reading I have concluded that all an amp should do is increase volume. Interesting that you suggest the O2 as a better amp. I use it and am amazed by the SQ of this combo with the 600's. Sometimes wonder if spending hundreds on more exotic amp would be beneficial though.