yep, hes right. I use my canalphones (akg k323 xs) at home. I seldom use them outdoors.
but the sennheiser only has a 18 ohm impedance if i'm not wrong. mobile devices should have no problems driving this pair. adding amps will make the headphones sound veiled. trust me. I tried that once.
and dac? unless you can hear distortion and hissing I really dont see a point in getting an external dac. as phones are designes to play clear sound without hissing.