I just picked up some headphones and DAC (digital to analogue converter) relatively cheaply on Amazon. Man, I have been blown away quality and depth of the sound.

Headphones: Sennheiser Momentum 2.0

DAC: Dragonfly red

I'm using an old Samsung galaxy S4 as the source with a 128gb SD card, a bit-perfect player call "USB Audio Player PRO” and FLAC files. The Dragonfly is outstanding. It doesn’t need an additional power sound and is light on battery. Just sounds lively and engaging. Completely portable.

It cost about $420 USD total. It is a lot, but so far is the best money I spent. I actually demoed a bunch of configurations and they all lean toward several thousand for a headphone DAC/AMP combo. The Dragonfly punches well above its class.

Just a great simple combination.