The Sennheiser HD 560S are slightly better headphones for neutral sound than the Beyerdynamic DT 770 PRO. The Sennheiser are open-back headphones with a much better passive soundstage performance. They're much more comfortable and have a more consistent bass delivery. On the other hand, the Beyerdynamic have a closed-back design.
The DT 770 M is a special version of the DT 770 PRO with 80 ohms. It is made for monitoring in extremely noisy environments. The DT 770 M offers significantly better ambient noise attenuation due to it's higher headband pressure of the earpads, Fiio E10K should drive headphones up to 150ohm according to the manual. I know lots of people say they can get loud enough with 250ohm but in my own experience 990 where unlistenable with it. Anything after 1 o clock on volume would distort quite bad. So they would kind of get loud but highs just spike up and bass would recess.The DT series, especially 770's as opposed to 880's-990's have a bit more of a predominant low-end response, but its still very clear and easy to reference on monitors to place your low-end in a mix. Post Note: Massively successful producers and mixing engineers SWEAR by their own choice of headphones just as I swear by my DT-770's, and a lot
LOYr.