My portable system I use at work often is a Samsung Galaxy TAB S2, running USB Audio Player Pro, with an Audioquest Dragonfly Red USB DAC/amp, with a headphone of the day. Almost the entire library (over 2k songs) is FLAC ripped from my CD's and on a 128GB micro SD (and only takes up 47GB).
Run the Dragonfly Red (DFR) at 100% volume and it goes into line out mode, and outputs well to a separate audio amp/system. The DAC is pretty phenominal in the DFR, playing through my HT system in analog bypass mode to a NAD C 275 BEE stereo amp powering my mains, the music has NEVER sounded so clear, dynamic, and impactful and it normally comes through either a Yamaha SACD player or Oppo Blu-ray/SACD player through.
For Android devices, USB Audio Player Pro is probably the best audio player in existance even if not using a DAC/amp, and the DFR will compare favorably with ~$1k range home headphone DAC/amp combo. If you want portable hi-fi solution for ~$200 in addition to a phone or tablet, it's a great way to go - just need some good quaity powered monitors to produce quality sound. The DFR also works with Apple, Windows, and iOS devices. It powers most headphones with ease too, the volume level in the image below is at my normal ~75dB level, half way would be around 100dB, and full (all the way to the + symbol on the right hand side) ~125dB:
Short demo video of half volume...