With these headphones it's a different story - you are reaching the limits of the circuit, the DAP might only get loud enough in certain frequencies, but underpowered in other regions (typically bass), so it ends up sounding distorted.Ehhh try hooking up a headphone like the Abyss 1266TC to a decently powerful DAP - you can get the volume to where you need it but you can also clearly tell how underpowered it is
A decent DAP would have more than enough power to drive any iem. A deviation from unit gain really brings nothing more than distortions. I always try different gain settings on new iems/headphones. With iems on the DAP, and with headphones on desktop amps. Not once did I end up preferring higher gain settings. Actually once I accidentally toggled gain from low to med on M8 without knowing, and for the entire week I was trying to figure out what went wrong.