.... hearing any of the stuff that we would normally ascribe to an impedance mismatch. 

... If the apparent impedance mismatch isn't a problem...

In headphones one cannot speak of impedances having to 'match' which is a common misconception that I often read in forums/discussions.
In this case it is a voltage division thing that is the culprit (some say damping factor things as well)
If matching was needed a 16 Ohm HP would need to be driven from a 16 Ohm source and a 600 Ohm HP from a 600 Ohm source. 

Close to 0 Ohm amplifiers would technically be an 'impedance mismatch' with ALL headphones, as none of them are close to 0 Ohm in impedance.
A close to 0 Ohm amplifier, however, may not be the best 'match' with certain headphones in the sonic department, just like an amplifier with a 120 Ohm (or different value) output resistance may not be the best 'match' with other headphones.
I figure that's where 'impedance mismatch' phrase originates from ... technically an incorrect one and not applicable to what technical guys mean with 'impedance mismatch'.
Nitpicking perhaps ... but I like/prefer the usage of proper wording. 

Impedance mismatching can happen with:
video signals, digital signals, radio e.t.c. everything that basically has frequencies above 100kHz where the wavelength of those high frequencies becomes important in relation to the impedance of the cable and the load as well as the source impedance should be 'matched' to avoid reflections and too much attenuation or peaking of certain frequencies.

One of the two or three worst owner's manuals I've ever seen.

Interface isn't exactly what I'd call intuitive.

Haven't listened enough yet to say anything about the sound.

It's an iRiver w/ digital output was the initial cause for the interest.  Use lots of aluminum and make it expensive and there you go.  Playing hi-rez doesn't hurt it's appeal and maybe 160gb of SS storage capacity.

Did you read my impressions on the 901?  Bass is a bit muddy, everything else is fine but very expensive and no digital output w/o the dock which makes it even more expensive. It played everything I threw at it except for one 24/88.2 out of three.  Early FW Fang said.  Basically my Leckerton amp is still better than the current built in module he used at CES but much much better than the original stock unit in the 801.


If I had to guess, the Vintage switch applies the same or similar Butterworth filter for 'analog' sensation as he applied to the 601/602.

Close to 0 Ohm amplifiers would technically be an 'impedance mismatch' with ALL headphones, as none of them are close to 0 Ohm in impedance.
A close to 0 Ohm amplifier, however, may not be the best 'match' with certain headphones in the sonic department, just like an amplifier with a 120 Ohm (or different value) output resistance may not be the best 'match' with other headphones.
I figure that's where 'impedance mismatch' phrase originates from ... technically an incorrect one and not applicable to what technical guys mean with 'impedance mismatch'.
Nitpicking perhaps ... but I like/prefer the usage of proper wording.  .

Assuming the amp can source enough voltage, you can easily toss in any resistor you want. Therefore, 0 Ohm is more adaptable, assuming the amplifier has enough voltage gain.
With some headphones, this will drive them like a voltage source instead of current source. (important in case of balanced armatures mostly, again, the finicky beasts they are)