i think FLAC is benificial for older music. I have a collection of Eric's Clapton in FLAC and i can def tell a difference in his live disc compare to 320.
I run a DAC , tube amp and headphones.. with FLAC the background noise and instrument sound more lively compare to 320... but again, everyone's hearing is different and subjective.
If it is for just regular playing outdoors, just use 320... if it is for @ home quiet listening time, I would run FLAC