Ok, just had a listen - not an exhaustive test but a listen straight out of the X5.3 and also via the Cayin C5 amp. The M50Xs work very nicely, possibly the best I've heard them. Plenty of high frequency definition without harshness, low end is there though perhaps a little understated. Soundstage is as reasonable as you would expect from these cans and overall presentation has a nice quality of air to it. They do benefit from the additional amp (C5) but I was pleasantly surprised with the result running straight out of the Fiio. You don't need an amp, but you'll probably want one! I listened to Chan Chan (Buena Vista Social Club), Matangi (M.I.A.), Rock Steady (Aretha Franklin), Pink Panther Theme (Boston Philharmonic (I think)) - all sounded impressive with the Pink Panther Theme being presented especially well.
HOWEVER - and I know this is not what you were asking, but I'll give my slightly off-topic advice anyway - the M50s pale in comparison to the LZ A4 and if you were an IRL friend of mine, (depending on your current source) I would advise you look at buying new headphones/earphones before upgrading your DAP. I loved the M50s when I was using them but since I have bought more capable gear I don't use them much at all. In terms of soundstage, bass presentation, detail, layering, entire experience - the LZ A4s walk all over the M50s - that's an IEM slamming a full size circumaural headphone.
If you have no interest in upgrading your cans, I can tell you that you will enjoy the X5III paired with the M50Xs.