I’ve never used a standalone headphone amp outside of a 4 way studio splitter/amp which is well beyond what you’d need. Nowadays I use the amp built into my mixers which, again, is probably outside of your needs. Most breakout boxes (SB Audigy) and usb amps (astro mixamp) can decently push high end studio headphones… decently meaning they can drive them with some colour to the sound but they simply won’t reach their max drive capacity. Same goes for my iphone/ipad and laptop.. they will push a decent, clean signal to my ATH-M50s, they just don’t get very loud.
That’s really the biggest problem with most products that only draw power from the USB bus, they just won’t be able to REALLY push good headphones, but they can get the job done. So, if you already have a mixamp, audigy breakout box, or anything else that’s piggy backing power from you PSU in some way, the Fiio might be redundant in terms of power output.
It’s biggest appeal is that you’re getting coax (digital) audio, which is super clean and has a virtually non-existent noise floor in most cases. This means you can probably push this thing to 11 and still have a crisp clean noise floor.. I can’t speak to the quality of the amp at max level, but your zero-signals will be zero as fuck ;)