1.) I personally don't believe in burn in and you can always just burn it in yourself.
2.) the general consensus on the he-400 thread is that the he-400 has improved bass sound quality (detail/texture/impact/extension) compared to the dt990 and there are people saying other sound quality improvements on the he-400, but i don't remember what exactly was said. of course, i have not been to the dt990 thread to see what they said and I have not had a chance to directly compare the dt990 against the he-400.
3.) however, from my personal experience of owning the he-400, I will say they are amazing sounding headphones and their price is currently down to $300 as hifiman is planning on releasing the new he-400i at $500 and the he-560 at $900. I personally don't think you can get any better performance for price at this price bracket up to even $500. (if you are looking for a darker sound signature & love amazing bass).
4.) the he-400 will run fine with the e18. it's pretty low impedance and high efficiency. you can actually achieve listenable volumes directly out of a 3.5 jack. some people do say that sound quality improves as you scale up your external equipment, but basically almost any amp will be able to drive the he-400.
Edited by money4me247 - 2/8/14 at 2:21pm