I haven't tried the Edition XS, but people seem to be like them and rank them as a good performer for the price range.

Coming from Focals and Sennheisers, I had the same feeling as yours about Arya Stealths. Keep in mind that they use a different driver technology and obviously have a different sound signature. Also, in your case, you're more used to the closed-back sound of m50x and wh-xm4. I don't think changing your amp will help much. Just give them some more time so that both your headphones and your ears/brain burn in and get used to the sound signature 😉.

Again, not suggesting to change your amp, but if you would like to try them on something else, give ifi DAC/Amps a try because they're less sharp as compared to the FiiOs and you may get a fuller sound from the XSs.

Happy listening anyways 🙂