So what's your theory about vanishing of hiss by adding an amplifier using the same signal path? My theory is that it can only add noise, not subtract it – unless it has a noise filter, e.g. in the form of a low-pass.
I don't believe in the «much better»; not even the Chord Hugo sounds much better than my X5 to me, let alone my X5 II. BTW, this thread is about the X5 II, so you may want to post your comparison in the A17 or if you need to in the X5 thread.
I don't adapt the EQ curve to genres or recordings if that's your interpretation. I have assigned fixed EQ curves to specific headphones/earphones meant to provide an over-all flat frequency response to my ears. I'm glad that the FiiO players allow that.
Is this question addressed to me? I want my headphones to output a signal as neutral as possible to my ears, so what I care for is the result from the whole chain, not a linear signal from the DAP. The weakest point will always be the sound transducers; equalizing their obligatory nonlinearities is a must to me. I rely on the fact that a linear (over-all) frequency response equates to a linear phase response – although of course just a raw approximation is possible,...
I'm aware of that, on the other hand personally I'm not overly dependent on system matching since I equalize. But my point was rather that a portable system is limited in sound quality to begin with, so adding gear means adding weight and bulkiness in the first place, leading the portability approach to the point of the absurd.
Yes, I'm perfectly happy (since I'm not expecting FiiO to implement a parametric equalizer, which would be too much asked for). I especially like the 0.2 dB steps – only undercut by the AK380's 0.1 dB steps. It's actually impossible for the X5's EQ to produce distortion (clipping), since it buffers the possible 6 dB increase with corresponding bits, so I wonder what you were experiencing.