I received an Anker Powercore II 10000 today.
It has about the same dimensions as my RPi 3B+ case.
I’m using this RoPieee with a Meridian Explorer² and MrSpeakers ETHER C Flow headphones.
Much cheaper than a DAP and good enough to enjoy good music at home while staying mobile. 
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Nice solution 
I got a great deal on a Chord Mojo+Poly so I use them for mobile home listening on headphones 