I have the FiiO X5 Gen III, and i'd agree with the no lock on the scroll wheel.
Sound is good for what you pay for, again, im only using it for transport or out in public. never use it at home because i have a better setup.
But its over-all very good. i've never had trouble pushing mid-tier headphones, not sure about super power hungry headphones but i'd imagine it wouldnt push it very well.
It does have balanced so thats up to you.
I'd look into maybe the X7 Gen II from FiiO... looks like it could push much harder stuff.
X7 looks perfect!! As to does the X5iii unfortunately I just can't afford it haha. That's why I was looking into these three as they are all sub £300.