audio quality is present in both th e M6 and the sflo2, my preference is the M6 as it just has a better personality for me, the sflo2 is very clean and raw
for eg, when i first got the sflo2 i did a blind test with 2 mates, one has a great ear for sound the other, deaf as a post

, played same song on both players, hid them under table and let them listen, whilst i swapped between the 2, back and forth - both preferred the M6, their comments were that the sflo2 sounded "tinny" compared to the full, evolved sound and personality of the M6, they liked the sflo2 and one, with good sound appreciation, appreciated where is was coming from and the clean sound it produced, but they did say, and i somewhat agree, that in doing this it stripped out some life of the music - again, its all very personal
i always fall back to the M6, no player has beaten it, not the sflo 2 or the samsung P3 or any of the other players i have owned - M6 is all round excellence for me and ticks most of the boxes
meizu is dead on the m6 now, you may find a few but production is stopped - they exited the race, apparently they were producing a faint trickle for a while but i am sure its stopped now