The E9 is 130 dollars and a neutral amp and has a crapload of power. You won't even hit 50% volume.
I think people here blow things way out of proportion. I have friends who have owned various ohm versions of the 880s at the same time. One owned the 32ohm and 600ohm. He couldn't tell a noticeable difference in sound. The other owned a 250ohm and 600ohm. He also couldn't tell a difference. All that was different was volume levels.
I myself owned the 32ohm 880, and the treble is nowhere near as aggressive as my 990/600.
But we all hear differently. All I know is that people buy stuff out of fear for unjustified reasons. I myself wish I'd have bought the 250ohm when I got my 990s as it was 40 dollars cheaper, and the one person I asked who tried both couldn't tell them apart sound wise.
Placebo effect, and buyer's remorse is all I see when people go straight for the 600ohm. At least it makes sense with the 770s as they are undoubtedly different, even with the same ohms, they have varying versions.
And the 32ohm 880 was awesome with the Mixamp, though I still felt that it needed an amp.
You bought the 600ohm because you heard it was better. Ironically, this was while they were tested with equipment that drives them to a higher degree than the Mixamp. For your use, you made any benefits of having the 600ohm obsolete, and would have probably gotten a better boost in sq with the lower ohm 880s as they need less juice. So even with the mindset that the 600ohm is better, you weren't going to gain those particular benefits (if any) with your setup.