I suppose one coould measure the AMP in parallel to the more real load of certain reference headphones, just in case the design is specifically broken.
(e.g. at specific capacitance, impedance and/or phase)
I wouldn't care too much about ringing anyway - many headphones ring far more on their own.
In other words - these small amounts of ringing should be impossibly hard to notice on any headphone.
However, they might cause oscillation issues in other parts of the circuit (assumiing there are any) - that's where proper design is required.
You'd need something like really high end balanced armatures to even measure that. (Just see the frequency of the ringing; I assumed the square wave is 30 Hz, yes?)
Or perhaps very high end electrostatic or orthodynamic driver - and that's really, really far yet from being able to discern this effect with both synthetic and live music.
This is unlike THD+N or frequency response issues, since those effects are straight additive.
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They don't all sound alike. Repeating transparency over and over is only detracting from this project, people can most likely get the same price/performance ratio for cheaper from China anyway.
DO IT THEN. Without all the necessary, expensive and validated (as in precisely described) setup of a double blind test, all you do is trolling.
And not exactly for cheaper - the closest product out there that is about as good is probably FiiO E17 - and it's twice as expensive, has more features, but lower power output. It won't be able to drive the least sensitive (highest impedance) headphones reasonably.
The other amps FiiO has made (I've singled out this manufacturer because they're one of the cheapest yet good amps) have non-transparent amounts of noise, minimally noticeable but still there.
Yes, most people can hear -90 dB wideband noise with well isolating IEMs. It's even easier to hear a peaky noise - sounding whine-like.
(Even highly isolating headphones don't have enough isolation and sensitivity to do this in the local environment for me.)
The -96 dB is about the threshold of audibility of wide-band noise - part of the reason it's been chosen for the CD Audio. Note that ATH curves show the tone curves which tend to be less sensitive by at least a few dB.
Note that I'm using 0 dB = 2 Pa - anything louder is really unhealthy for even short term. (Actually I calibrate loudness with multiple speech samples to sound properly loud, since I'm too lazy to measure this.)
However, it's a very mild artifact compared to frequency response issues caused by high output impedance (common issue), or compared to larger amounts of IMD, which sounds really harsh.
(Also what with many people are not measuring IMD right,)
IMD and (huge) jitter issues are what can be perhaps attributed to sound "glass-like".
