With all of the amps that I have reviewed, I don’t think there is any exception to the rule that the amp will produce a cleaner sound at low gain. The background is always blacker, instruments more distinct, less congested sound, and a bigger soundstage — all the result of using the amp at low gain.
When you’re on the low gain level, you may have to use your volume control at the high range, around 3 or 4 O’clock as opposed to a more “normal” range at 10 or 11 O’clock. Fear not as this is actually a bonus, as the potentiometer is known to perform better at close or near maximum, so you get an even cleaner sound.
With high gain, you get the opposite of the qualities I described above, but I can see why some people prefer high gain. Often the sound is more forward on high gain and bass punch tend to be punchier. Don’t ask me why that is, just something I picked up from doing reviews. It’s a more fun sound, at the expense of the clean technicalities.
Of course with some headphones I have no choice but to go with high gain, but when I have a choice, I set it at low gain.