I have never had that problem with my KRK KNS 8400 at any volume regardless of amp or dac combinations. Even straight out of Macbook pro jack or Sansa Fuze, I can get really loud but it never distorts. Even my E10 works just fine. Don't go for E7, unless you really need the portability, instead go for the E10, totally works fine with my KNS 8400, D2000 and even my HE-500. Under $100, they are the one of the best if not the best. Also based on the specifications noted, I find the 8400s to actually need some power to get dynamic. An E10 will do a good job with that, and you will easily enough power to make it fully dynamic.
They definitely are bright, but very slightly. As for the distortion issues, at first when I put these headphone on, I did notice this weird hiss presence (not referring to the floor noise or impedance/sensitivity issues) but just a weird electric thing with the highs, like a buzz. After a week of 40 hours ish, this definitely got better with break in and this also has to do with me getting used to their presentation. Out of all my headphones, it took me the longest time to get used to the 8400. However, if you actually hear pops and cracks, then there might be an issue with the driver.
Also if you have other audio equipment, try listening to them and keep switching stuff here and there and then come back to 8400 and do this for many rotation. This definitely helped me like the sound of 8400 more and get used to those highs.