well there is no carrier system in India.. I can get my hands on any phone here..I was even considering the note4 edge but then got stuck at the budget.. Its around 550 dollars for the phone n d dac or dapIf he's gonna lug around a brick, a DAC-HPamp that doubles as a powerbank would be a more sensible option if money isn't a problem anyway.
That said...why is the One Plus One your only option? I just assumed you already had it. If you don't then can't you look into getting a phone with a microSD card slot, if not also swappable batteries? I'm still using my SGS3. As a player it has some distortion on bass-boosted earphones and the volume adjustment steps are problematic for new albums, but NeutronMusicPlayer app fixed that easily - the first with EQ and the second with ReplayGain. Original battery it came with is dead but I have a couple of new batteries that came with an external charger (like the one for my camera), so I just swap the battery and charge the drained one outside of the phone.
What other Androids are available from your carrier?