Hi. I am currently using the Fiio E9 with a pair of dt880 600s and they are definitely being driven very well. I would not recommend powering a 600 ohm pair of headphones with the e7, though. It is noticeable that they are not being fully driven, but they still sound decent if you crank up the volume. I'm sure they would work fine for your 250 ohm dt770s though. You may find a pair of dt990 600 ohms to be slightly lacking, but I can't imagine any other portable amp doing a better job. If you do not own any kind of amp, already, I would definitely recommend this combo. I have not listened to any other amps in either of these price ranges, or greater, but they definitely improve my headphones tremendously. I use the e9 for my dt880s and the e7 for my iems and portable headphones. I do not have any problems with the transparency of the E9 and don't feel like the highs are over-accentuated. I hope this helps you.