I would take the m40x any day over the flight… it’s nice but not that good in my opinion. It’s relatively a W signature until swapping out the pads but it remains rather boomy with a lot of rumble and sub bass, clean vocals, and obviously increased highs. I wouldn’t call it by any means a higher quality sound though and the build quality is pretty poor as it’s just cheap thin plastic without all that much padding on it and hotspot on the top center of the head. It also has quality issues where the thing just will stop being able to be charged after some time and is one of the reasons I just got rid of mine as I have gone through two of them.
M40x over there is a subtle V signature out the gate with some slight recession to the mids post pad swap it can change it’s sound quite nicely. It becomes more of a Warmer signature when you put a sheepskin sort of pad on it. Quite nice, Plastics a bit thicker than the flights, properly portable, better quality sound, doesn’t look gamey