Output impedance tends to be most noticeable.
There's obviously noise performance, voltage gain and current capability. THD+N, IMD and jitter as of nonlinearities, but it's a really lousy amplifier that fails there horribly.
And in case of portables, battery life.
About FiiO E6, it is a decent amp if you don't require lots of drive power - ok for IEMs and many headphones. Not strong enough to drive orthodynamics or high impedance versions.
(But definitely can drive something 80 Ohm and reasonable sensitivity like Beyerdynamic DT1350, unlike many portable sources.)
It has pretty short battery life. Not sure if anything can beat it at its price level - it can definitely beat many amplifiers built into cheaper DAPs and cellphones and due to impedance issues, even some soundcards.
I can't tell if you're provoking me, or just......
The amplifier in the Fiio E6 is the same as in many smartphones, PCI sound-cards tend to use higher grade parts in the amp section.
The Teclast X19+ or X19HD can beat it at it's price level.
The noise performance isn't very good.
You missed perhaps the most important measurement which is sine wave delivery.
Aside from all this accuracy you're aiming for (the Fiio E6 isn't accurate, the OPA1611 is), electric guitar amplifiers are usually distorted and coloured yet people love their sound, and there are various types of videocamera lenses for different applications, just like different microphones capture frequency response completely differently, so you have to respect all this technology and equipment.
1 ohm output impedance and flat FR is really futile compared to the reality of sound quality.
I think everyone should spend their money on quality equipment and quality IEM's, then they can enjoy music more and appreciate sound quality more, not just parrot specs which are only small pieces of a puzzle.