I've been listening to the E9 with DT990/600 and DT880/600 - both are driven to volumes far beyond what I would consider "comfortable", even when the gain setting is on "low". This amp has plenty of power.
This is a very quiet amp. I connected its pre-amp output to my main home theater and turned up the volume on the E9 and my receiver to max, E9 gain set to high, input is provided by USB/E7 from my computer, also with volume set to max. I could hear a very faint hiss that changes according to the volume of the E9 - again, this is with the receiver's master volume set to +10dB. How loud was this? I dragged the volume bar in windows down from max to about 1/10 and let go - the resulting "ding!" hurt my ears.
I will leave listening impressions to others, but so far I've been very happy with it.
Jack