The audio output of an MBP or iDevices is not really good. And if you want to drive better (=high impedance) headphones, forget it.
I tried not to go too deep into the audio rabbit hole but good sound is a must for me, so I settled on:
For stationary (=desk) use: a combination of Shiit Audio (no joke) Modi 2 DAC and Magni 2 headphone amp. This gives me improved conversion (D->A) and I can drive the most demanding headphones (500Ω no problem).
If I am on the road with my MBP: nuForce uDAC 3. USB-powered, serious sound improvemend, but not near the Modi/Magni.
Software: iTunes is not really good (and can’t play FLAC, or did that change?). I use Amarra, an alternative is Audirvana. Even VOX has better sound than iTunes.
Home: still not decided what DAC I should get, so I am using the optical output from the Mac mini and that goes into my amplifier (2.0, not 5.1, 7.1 or whatever).