Depending on the headphone the biggest problem is the amp not the DAC. E.g. something like a hifiman Sundara or a Sennheiser HD 600 Series requires a bit more power then a cell phone typically requires. Looking at the specs for the M3k it looks to me that it is not really suitable for these kind of headphones. I like using slightly bigger mobile amps with the Sundara. Used to have a Topping NX4, which worked well, but died after 6 months, now switched to an ifi hip-dac.
obviously a "hifi" dac/amp combo should deliver enough power at 300 ohms. I would suggest buying one that delivers enough power. or are you concerned with battery usage?
just a sidenote: the sundara you mention have 37 ohm which is pretty standard and they should be easy to drive.
Except the fact that you need an additional device anyway, FiiO has an amazing battery life and you don't have to drain your phone, plus you can put SD cards up to whatever is available today. Which is necessary for big lossless collections.
