Most DAPs on the market are these: http://www.apple.com/ipod/ , or  these: http://www.apple.com/iphone/ , or these: http://www.samsung.com/us/mobile/galaxy/ or similar, none of which have digital filter options. The DAPs on that page that you linked make up a vanishingly small minority of DAPs sold.

It can indeed be a problem, but it should be relatively easily solved by lowering digital volume on your computer to 80-90% so the DAC doesn't ever see 0dBFS samples.

  I'm in a nitpicky kind of mood, so just to make sure this is perfectly accurate, redbook doesn't perfectly store signals at any level, it always adds some noise or distortion. That noise level can be well below even -96dBFS though if dither is applied intelligently, allowing encoding of...

I'd prefer my amp to not sound like anything. I want it to make the signal louder and do nothing else. Luckily, there are some good options that do that.

See, to me, that implies that the Mjolnir is worse, since sounding different than a pure solid state device (assuming the solid state device is well-engineered) tells me that the Mjolnir is adding distortion. When you said the Mjolnir was better, I assumed that meant there was something audibly...

What's wrong with the Fulla, out of curiosity? I haven't seen enough measurements to make a judgment about whether the Fulla should be audibly transparent, but the specs certainly indicate it should be.

The strange thing about that, though, is that for some strange reason, they can only hear that it sounds better when they can also see what it is. When they don't have knowledge of what is playing, it frequently mysteriously sounds exactly the same as other, cheaper gear (assuming competent...

To be fair, distortion is an important factor. However, it's by far the most detectable when testing via fast-switch A/B. Levels of distortion that are inaudible if just listened to in isolation can be detected if you have the contrast of an A/B. You can experiment on yourself here...

Planars fall in that "esoteric and hard to drive" category I mentioned, unfortunately, but the 400i at 93dB @ 1mw is not too bad. If we (pessimistically) use the 50 ohm power figure for the Fulla, that gives a peak SPL of 118dB. This is probably enough, assuming you have a gain structure that...

Burn in for the vast, vast majority of components does not matter, and gear matching only matters if your gear is so woefully poor that it is coloring the sound in some way. Good quality amps, dacs, and sources are completely transparent and will sound identical and interchangeable.   As for...

I don't have time right now to address everything, but that's far from a good video. It contains some misunderstandings in how digital signals are conveyed, as well as how signal timing is used to reconstruct the waveform. It contains a lot of handwaving, but zero actual measurements or examples...

High sensitivity improves compatibility with a larger variety of portable sources, and improves battery life during playback. I don't think any manufacturer of high sensitivity headphones expects you to just crank the volume all the way and listen to them that way.

Interestingly, even when converting to an integer rate, you don't just want to take every fourth sample. You can introduce aliasing if you do that, which can add very audible artifacts. What you actually want to do is lowpass it first, then resample, to ensure you have eliminated any frequencies...

There's no evidence at all that amp differences are audible, so long as they measure sufficiently well (to be fair, to truly quantify this involves a more detailed set of measurements than just frequency response).

If the specs are accurate, they really should both sound identical. What sounds different?

In general, most of the evidence I've seen supports it being the other way around - a subtle but perceptible difference in a fast-switch A/B will not be perceived as a difference at all in longer term testing.

Is there any proof of this "fatiguing" character not being audible in a fast-switch a/b? I'd definitely believe that you might not be able to tell which one would be the fatiguing one, but I would think that it would at least be audible that there was a difference.

I suspect they won't agree, but go ahead if you want. In general though, if you're designing an amp with the intention of the cleanest possible signal for normal headphone use though, you really only have one choice these days: solid state with loads of negative feedback. Anything else is likely...

Hell, I'd settle for different. That would at least show that it's doing something audible.

It means that whoever designed the amp either did not design it to leave the signal as unaffected as possible, or they didn't use good engineering practices when designing it. That's abundantly clear from the fact that it's OTL though, since that's a very poor design for high fidelity...

Sound quality can be affected, depending on how reactive of a load the headphones present (how much their impedance varies with frequency). As for amps, I don't know what you consider affordable, but I run an O2/ODAC with my computer due to atrocious amounts of noise and insufficient power from...

Well, since any decent amp should have an output impedance much, much lower than the impedance of the headphones it is driving (in order to provide audible transparency), that's not surprising. It just means that one of your amplifiers was not actually designed with optimal fidelity in mind.

Well, it's true that it's audible, and not subtle. I definitely wouldn't call it an improvement to the fidelity in any way though.

I'm rather skeptical of both your high and low frequency response claims. It's very likely that your transducers or something in your playback chain is distorting when asked to produce significant levels of either 6Hz or 21.5kHz, and you're hearing the audible distortion products, not the tone...

That's not really possible - longer buffers will never change the music, and shorter buffers will only change it if you suffer from a buffer underrun (which will manifest as a pop, click, or interruption in the sound). Reverb, "sluggishness", "energy", and the like are never affected.