Pros: Sound Quality, Compact Size, Value
Cons: only 1 headphone jack, no line out
I agree with pretty much everything the other reviews said, so I'll add how this compares to my Headroom Maxed Out Home (MOH).
First, a little on the MOH I'm comparing with the Jazz. The MOH is a great amp: toroidal power supply, OPA627s biased class A, quiet background (high S/N ratio), linear, neutral sound. Power limited to about 250 mW and 250 mA which is plenty to drive just about any headphone short of a a HiFiMan HE-6. The MOH sound is smoother and darker than most solid state amps, yet despite being slightly dark it has incredible detail; it is not veiled. I used to be a tube guy (Wheatfield HA-2 driving HD-600s, a match made in heaven) and the MOH is the amp that converted me to solid state. In its day, the MOH cost a kilobuck or more depending on options and was on the Stereophile class A list.
The Corda Jazz is very similar to the MOH; both are excellent amps and the differences are subtle. The Jazz sounds more neutral: not as dark, but not bright either. It's hard to describe, but "sweeter" seems to capture it. Sweet as in the sound of the strings in the 2nd movement of Rachmaninoff's 2nd played by Earl Wild on the 1960s RCA Victor LP. The Jazz is not as sickly sweet as that, the Jazz is too neutral for such euphonics but compared to the MOH its voicing hints in that direction just not nearly as far. Both are very neutral amps overall, but the Jazz leans sweet where the MOH leans warm.
Compared to the Jazz, the MOH bass is a just a touch emphasized and wooly. Put differently, the Jazz bass extends just as deep but seems a touch more crisp. The MOH bass adds to its overall warmth and this is almost surely a contributing factor to the Jazz sounding less warm. It's almost impossible to describe this difference without exaggerating it; it's a subtle difference and I can't say I have preference; it's like two equally amazing yet slightly different presentations of reality. Both sound great and which sounds better depends on the recording. My LCD-2 (2014 Fazor model) headphones have deep linear bass extension and don't need the exaggerated bass so the Jazz pairs nicely.
In the extreme high end (15 kHz +) , the MOH rolls off just a hint while the Jazz seems to maintain perfect ruler flat response. These frequencies are well above what would constitute "brightness", contributing more to a sense of space than of sound, they aren't in many recordings and even when they are they're hard to hear. I can't be completely sure this is exactly what I perceived though there was definitely a subtle difference and I believe this is what it is.
Both amps have a headphone spatial processor, though the Jazz's is clearly better - more refined and subtle. The MOH dates from the late 90s and had one of the first spatial processors. It does work - improves imaging and reduces listening fatigue - though it bloats the bass (due to mono summing) and attenuates the upper mids / lower treble (due to comb filter effects). The MOH has a "filter" switch to give a 2 db boost around 1-4 kHz to counter the comb filter effects. I never used it for serious music enjoyment, only for casual listening and movies. The Jazz processor is a whole 'nuther thing. It does everything the MOH does (though it does it a little less, and here less is more), but with neither of the MOH's drawbacks and none of its own. It doesn't change the overall tonality or clarity of the music; it only opens up the image and reduces listening fatigue. Its effects are more subtle than the MOH, apparent mainly on natural recordings of acoustic music, which makes sense give what it is, and that's most of what I listen to.
Regarding how they work: The Headroom processor is simple: it takes some of the L, mixes into the R with level reduced and phase shifted. The Jazz processor is more sophisticated and selective. In Jan Meier's own words: http://www.meier-audio.homepage.t-online.de/crossfeed.htm
The Jazz has an adjustable gain switch that changes the level about 16 dB. I believe it swaps the metal film resistors that set gain across the op amps (instead of using fixed gain and attenuating the signal before or after the op amps), so it should be sonically transparent in either position. Low gain might measure cleaner (lower noise, wider bandwidth) but the differences are probably so small as to be inaudible and negligible. Any differences I thought I heard between low and high gain mode disappeared in level matched comparisons. The difference makes about a 1/4 turn of the knob. This enables it to drive pretty much any headphone under the sun. My LCD 2 headphones work well in both high and low gain modes, and after measuring the amp and speaking with Jan Meier, I use the high gain mode. Note: my LCD 2s are about 3 dB more efficient than Sennheiser HD-600s. The high gain mode is intended for normal use with full size headphones; low gain for earbuds, IEMs and high efficiency stuff made to be driven by phones.
The Jazz has a very nice volume knob. The way Jan describes it, the potentiometer is not in the signal path, but it triggers relays to set the gain like a ladder stepped attenuator. This should be sonically transparent with perfect L-R channel balance at every setting. It's a very nice touch and simply unheard of in an amp at this price.
The Jazz output is neither balanced nor unbalanced, but something Meier calls Active Balanced Ground Driving. It is compatible with unbalanced headphones, providing better noise rejection than standard unbalanced outputs. I wish more engineers would show this kind of simple, creative approach based on sound engineering. But it is not compatible with unbalanced analog audio inputs, so don't connect the headphone output of the Jazz to a preamp or similar analog input. More detail in Meier's own words: http://www.meier-audio.homepage.t-online.de/grounds.htm
The background noise on the Jazz is very low, but not quite as low as the MOH. On low gain at max volume it's dead silent. The loudest my ears can tolerate is around 3:00 and normal listening is around 12:00 so it's dead silent at actual listening levels. But on high gain at max volume there is an audible low level buzz/hum. The MOH gain is similar to the Jazz in high gain mode, and the MOH also has a low level buzz/hum at max volume but it's about 3 dB quieter. Either way, both amps are dead silent even at loud listening levels.
To summarize, both amps take you close to the sound of the live mic feed but in slightly different ways. The MOH gives a slightly warmer, darker, smoother presentation. The Jazz gives a more neutral presentation with a touch of midrange sweetness. Ultimately I think the Jazz is closer to the live mic feed, but the MOH is a slightly more "musical" interpretation of it. No "better", no "preferrered", fully equal just different.
Even if I ignore the modest $415 USD price, the Jazz is as good an amp as I've ever heard. The MOH cost nearly 3 times that 15 years ago and would cost even more if produced today. Given the price, it's the biggest pleasant surprise I've seen in audio for a long time and the best value I've ever seen.
Addendum: I recently measured this amp, results here. It has excellent measurements to match its excellent sound quality.