My honest opinion. The A&K 120 is too much money for a file source.
I would say buy a cheap file source. Then buy some more headphones. I have the Mojo and the Grado 225e. I had to EQ the highs out of the 225e to get more balance to the sound. Then I heard the mid of the music more richly. ... Recently I bought the AKG N40. They N40 are balanced, full sound, low bass extension, smooth clear and even treble. No need to EQ the AKG N40, and they sound better than the 225e. I never use my 225e now. ... I was lucky because the AKG N40 I bought were on sale. Normal price £350, but I paid £250. (The took about 20 hours to sound right.)
With not having to EQ for the AKG N40, I can use them on anything. My iPod Shuffle, my Mojo, my Hugo 2, or my portable pocket DAB radio. They have the same gorgeous sound signature on every music source. The Hugo 2 is best though, of course.
I do not mean you should buy the AKG N40. However I do mean maybe you should consider that you would be happier with better headphones. I recommend the AKG N40 to anyone, but it is always a matter of individual choice and preference.
In the Hugo 2 thread I was asking for ideas for a file source. It was suggested to me that iBasso Dx90 was a good source. (Replaceable battery, USB and coaxial output, and up to 2TB SD card.) However the Dx90 is an older model and you would be looking for a used unit. I think the Shanling M1 is ideal. Any nice good value budget DAP is a good idea.
I do not really believe there is too much difference in file source. I have a Sony ZX300 DAP, and I hear no difference between ZX300 and my computer, both on USB. I would also like a cheap DAP so that I can use my ZX300 as a DAP. Then use the another cheap DAP as a file source. Or a netbook.
Optical is probably the best, but a clean battery powered DAP will give you a good sound. Make sure you get bit-perfect. .... What is wrong with your LG V20. It has good native sound, it has OTG, and I think the battery is replaceable. What more do you need.