I'd like to reinforce the importance of amplication power needed for these headphones, by way of anecdotal evidence.
I posted before about the A2C sounding compressed, shrill and "noisy", difficult to listen to at average to high volume. This was to the point of considering returning them, at a great financial loss (I live in Australia and ordered direct from DCA in the US). I was using a Topping DX7 Pro as a DAC/amp combo, which has an output of
637 mW at 33 Ohm.
Then, I bought and received the Topping A90, which has an output of
2 W at 33 Ohm. Oh, what a difference in sound. The headphones now are open, dynamic and clear. I can listen to loud volume without fatigue, and I just want to listen to music more and more, whereas before I didn't have this urge at all and all I felt was disappointment.
Considering both the DX7 Pro and A90 have low distortion figures, I can only imagine this difference is mainly due to the power.