Here's my (personal) take on X3II and iBasso DX50. I've owned DX50 for almost a year, X3II for about a month.
Pushing aside the features, settings, size which I must give to FiiO the sound is easily different enough to own both if possible.
X3II sounds technically stronger, areas like refinement, separation, coherence across the frequency range are a step above DX50, hearing samples during busy passages are easier to make out. DX50 can sound slightly confused compared to X3II on this technicality. The FiiO also sounds more musical and smoother overall, has a great sense of enjoyment for casual listening.
And that's where my second hand comes in. While I find X3II stronger overall, even areas like bass being tighter, controlled and showing good speed (often related to driving power), it sounds a little blunt or dull compared or missing some vibrance around the mid-range. I pick this up with vocal tracks, that I'm missing the absolute clarity I find in the iBasso units.
Moving on while DX50 technical areas aren't as competent (expect soundstage which is slightly wider to my ears) when it comes to that separation and strong refinement I'm willing to sacrifice this for higher clarity and detail, or more to the point effortless presentation, so you're kinda stuck in a catch 22. Technical / vibrancy, which do you prefer?
I want to hear what I like most, while I love (I really do) admire X3II's technical aspects it simply falls short in the area that I require first in foremost.