We are talking about volume Pot noise which is noisy regardless of gain when you turn it even with music playing. While at high gain and no music playing my T1 and DT235 has noticeable hiss while no music is playing. it's very faint in T1 but more noticeable on DT235. Med and low gain are quiet but it's a moot for 600 ohm headphone.
Again, no noise whatsoever on my m8, but when using the back switches or turning it on and off. Anyway I understand what you say when you feel the m8 a little anemic, I had the same feeling yesterday when I used my vali after a couple of days of m8, but the switches behind helped a little to adjust what I felt was missing.