An excellent dac-amp at this price
I use the Fiio K7 with Hifiman HE400se headphones and it drives them well (I usually listen with the volume on low gain and the knob between 1 and 3 o'clock). The volume control is done in the analogue domain, so changing it has no effect on the signal quality. Some people remark on a lag between turning the volume knob and the volume actually changing, but this has not been a problem at all. I especially notice the soundstage (instruments are easy to locate when listening) and the ability to reproduce a large dynamic range cleanly. I listen mostly using Roon upsampled to the dac's limit (on USB, 384kHz-32bit/DSD256), so adjusting the dac filters isn't really necessary, still it would be nice to be able to fiddle. Very happy with the purchase.