This is a review and detailed measurements of the Project Pre Box S2 Digital USB DAC and Headphone Amplifier. It is on kind loan from a member. The S2 costs US $399 including Prime shipping from Amazon. I reviewed an early unit before I have my new audio analyzer so request was made to measure it again so here it is.The S2 is a compact box with a color but grainy LCD:Dedicated filter and Menu buttons make navigation easy.The back panel has the usual connectors:Since I tested the S2 using its USB port, I did not have to supply external power to it. Lack of external supply likely limits its headphone output level.The S2 supports MQA which is still rare in lower end audio DACs.The S2 uses ESS Sabre DAC which has a distortion compensation feature. The S2 is the only DAC I know that lets you turn that feature on and off. You will see its effect in measurement section.Let's start with our usual dashboard:Even though distortion compensation is ON, we still have rather high second harmonic which brings SINAD which is sum of distortion and noise relative to main signal down fair bit. The ranking straddles two of our buckets:This is not a good first impression for a product in this price range. I thought the news would be bad from here on, but that was not the case. The S2 produces a very quiet background:This is just a few dBs shy of state of the art.Good news continued into linearity measurement:And jitter & noise measurements:The low noise floor gave the S2 a leg up in the IMD versus level in the noise-dominated portion of that graph (sloping down):As levels increase though, distortion takes over.ESS DAC implementations tend to have a mid-level bump up in the above measurements. Manufacturers have lately been able to conquer that. Interesting that the S2 despite being an older design, is completely clear of that issue. So the secret was known earlier among some designers.Multitone results are more hopeful than SINAD:THD+N versus frequency swings back to disappointing side of the fence:Filter response is standard affair:Turning off distortion compensation reduces performance substantially:Can't imagine why you would want to turn this feature off.Let's start with our noise measurement at just 50 milivolts to see if the quiet DAC noise floor shows up here:Looks like the eye was taken off the ball here:Sensitive IEMs will likely have some amount of background hiss at elevated volumes.Power into high impedance load of 300 ohm is quite low:Yet again we see elevated noise levels. The headphone amp is definitely noisier than the DAC portion.Ditto for 33 ohm load:This is very low power level. Hard to imagine a desktop product shipping with such low output level.I thought maybe output impedance is high but it is not (thankfully):Not feeling good so I skipped listening to the S2. The numbers speak to inability to reach any decent level of loudness without distortion.There are some really bright areas in performance of Pre Box S2. Its DAC has excellent noise floor, great accuracy in the form of linearity, and very low jitter. Distortion is high though for the class. That may not be an audible thing but the very low output levels of the headphone amplifier certainly is.There are many good choices in this price range and lower so I can't recommend the Project Pre Box S2. But if you find a used one, pair it with a good headphone amplifier and you would be good to go.--------As always, questions, comments, corrections, etc. are welcome.Was not feeling good per above and went to the doctor. He said I have MSD. I asked him what that was and he said:. I asked him for the cure, he said it would come in the form of donations form you all. Of course the son of a gun took more money from me, making my symptoms even worse! Please help dig me out of this disease byusing : https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/