Since I am on furlough, I have taken the opportunity to study this DAC in a bit more detail and I share my observations.
The SMSL design, as commented by a couple of people only seems to utilise two of the four channels available on this DAC chip. I am simply amazed at how much performance has been extracted from half of the DAC chip as evident from the measurements. In full output DAC mode, I have not seen better SINAD or Dynamic Range figures from any implementation of any manufacturer's DAC chip. I believe the name of the game in DAC designs is to retain as much of the performance of the DAC chip as possible in the external components required to make it functional; this performance is very fragile and easily compromised in the design of the PCB and the external components. SMSL have done an absolutely brilliant job in retaining simplicity and preserving the performance of the DAC chip.
I also observed that there appeared to be electrolyic capacitors in the signal path. These are DC blockers and are possibly provided to protect the subsequent amp under normal and fault conditions. Many "audiophiles" would shrink in horror from this preferring DC coupled or at the very least more stable and linear film/polyprop etc. capacitors. The measurements appear to show no negative implications on linearity of the DAC so this is one for the "believers".
With regard to the output impedance issue, I agree with a number of people who have stated that this is a non-issue. This issue only applies to the balanced connections, the single ended outputs are buffered through additional opamps which also act as summation/LPF filters. For balanced outputs, because the maximum output voltage is around 50% higher than the "standard" 4V there is a tremendous amount of scope to compensate for any additional volt drop across this 1240 Ohm output impedance. As I have stated before, the output circuit is a verbatim implementation of the datasheet recommended circuit. To put this into perspective, the ESS Sabre information recommends resistors in their balanced passive LPF circuit equivalent to a 4000 Ohm output impedance.
Finally, stating the obvious here but whilst the testing at 4V with the resulting lower performance is a fair way of comparing different DAC's with different output voltages, however in use, in pure DAC mode or indeed at full output, it will be possible to benefit from the higher SINAD and dynamic range available and please correct me if I am wrong, there is simply no other commercial DAC available with this level of measured performance?