Really clean meaning what exactly? at what price point would a DAC be indistinguishable in a blind A/B test to a $10,000 one? I'm not trying to be pedantic I just think it's important to ask these questions.
Many $10,000 DACs are measurably inferior to a $9 Apple USB DAC. Start be crossing all those off the list.
The Apple DAC is actually insanely great, the best price/performance ratio of any bit of new audio equipment ever. They are not particularly mechanically well-made, though, and can easily be broken from repeated bending.
In a real world level-matched double-blind ABX test, $9 wins.
This is a great question and I'm glad you're asking it. Once you get up to 90-100dB SNR I think most people would say you're well into transparent territory with decent headphones. 120dB is absolute state of the art (and the limit of must measurement gear!) but the difference is academic at that point.
Those mini dongles are pretty good, although they're not necessarily exactly world beaters. They're DACs + amplifiers and their amplifier sections don't exactly deliver a lot of current. But for e.g. desktop use you can get a really good DAC + headphone amp combo from Schiit, JDS, Topping, or others for $200ish.
Minimal distortions, but especially one thing: less reaction to external RF noise. Remember living in the early '00s, back at a time when cellphones were barely regulated? Every time someone got a call, you could tell two seconds earlier because of the characteristic buzz noise in the radio.
Another part is less reaction to unclean power. "Thanks" to everyone and their dog using Powerline ethernet, having a clean power supply is ... difficult. You won't notice that in your audio setup unless your neighbors are all blasting the power lines, but if you're working with high-frequency stuff it's definitely noticeable.
