I use the hp50 with my iPhone through the line out with an E11 playing Spotify (320kbps) and on my Mac through the headphone jack also playing Spotify.
Would the headphones benefit from using FLAC/ lossless files instead of 320kbps, and would i be able to tell a difference?
I would think it very difficult if not impossible to reliably tell the difference. Most people who think they can are wrong and have never attempted to prove it using blind testing. Try it yourself, put the same song on your device twice, named differently, one lossless, one 320. Have a friend help you, get them to do say 10 trials that they randomly decide which file plays and for how long (2 minutes per trial for instance). If you can hear the difference around 75% of the time that means you are reliably hearing the difference, if it is 50% or less, that is explainable by pure chance.