The reason why people think their phone can't drive the Sundara I think has something to do with the automatic low/high impedance switch some of them have when detecting what's connected to the audio jack. Some phones change their output from low impedance to high impedance and some even to "auxiliary" without the end user noticing. For example, the LG V30 has a low impedance and high impedance mode that are turned on depending on what's connected to the jack. I have this phone and the Sundara doesn't get to a decent volume when connected to it. The phone detects it as a low impedance audio device and doesn't try to put out the full juice. I have to fool the phone into thinking it is a high impedance device by using an adapter first and then connecting the headphone to the adapter. The phone switches to high impedance mode when it detects something that is 50 ohms or higher. So yeah, I think different phones have different implementations and thus why some people complain. The Sundara regardless of how efficient HIFIMAN says it is does requires a bit more juice than other phones with the same efficiency specs. I have some old mp3 players I had lying around and I can't get it to sound high enough on them.