So I just got my D30 and X7S and tried them with my m50x and hd6xx single ended. At medium gain setting my m50x get pretty loud around 12 o'clock . with 6xx I have to go to 2 o'clock and I don't find the sound coming out from it satisfying. What can be wrong? Is it just my mind ? or what?
And also there is a notable hiss with quiet tracks with both headphones but it goes right away when I pause the track is it in the recording or the amp doing that?
How loud do you listen? Distortion does tend to go up with high power usage, as you can see from Amir's graphs... but that is at what should be really loud levels, given the power output. Are you listening at more that 110 dB? That's the only way I could see you hitting the kind of power levels that would cause the increased distortion with the HD 6XX.
Otherwise you may simply not like the amps signature or frequency response, with some part of your brain picking out things it doesn't like. If you can listen with your headphones on another adept source and do it soon enough to have a clear impression, you could find out if it's the same. Another, more grim, observation is that you may simply not like your headphones actual sound now that they are being powered properly. Depends on your prior experience with them.
Also, I might be wrong and someone correct me if I am, but as I understand it a Class A headphone amp (sort of an "always on" design) wouldn't be causing the hiss you are describing. The fact that it pauses along with the track is probably from the recording, not the amp. There are certain devices and amps (generally low quality, built-in type solutions) that hide hiss by only enabling the amp portion when music is playing, making it difficult to isolate.