I particularly like the X5 with Earsonics IEM and the HIFIMAN. With the X5, the Momentum sound seems to be thinner with less definition. Heaven VI is good too as it adds smoothness without weakness.
IMO, my Earsonics and HIFIMAN are very close to the sound I like: pretty neutral, big soundstage, black background, good PRAT.
The best headphone / IEM does not exist as it depends the kind of sound you like.
Oh sorry, what I was trying to gauge was how much of the SQ improvement you heard is coming from the lower output impedance of the X5 compared to whether or not the X5 actually has a higher quality sound