Ironically, I'd be pretty hard pressed to believe you could tell the "quality" from a Fiio DAC compared to an iPhone
Don't get me started on the Bluetooth headsets. For music they are ok but there's really basic interval training that I score 100% on with the DAC and like 80% with Bluetooth. Consistently.
In something like a year I went from "telling apart major and minor thirds is black magic" to being able to sing the major scale right like 60% of the time and I have an 80% success rate for all intervals up to a fifth. It did improve my piano improvisational skills by a lot.
https://www.ifixit.com/Teardown/Apple+Lightning+to+Headphone...
I only started bothering with discrete amps when I picked up headphones with a 600 ohm impedance.