Review: Pro-Ject Audio Pre Box S2 Digital – upstream success

Pro-Ject Pre Box S2 Digital



After testing every filter permutation, I’ve settled on the slow roll off and Pro-Ject Audio’s Best setting. Why? I favor the combination of overall volume levels and slowly softened highs. This tendency is something new. I used to like the harder, the more detailed, and the faster roll offs. And then I turned thirty five.

In evaluating the S2 Digital, one can’t simply pronounce a sound signature without first running through 8 filter sets, distortion compensation, and a variety of inputs. The S2 requires time.

And the time I’ve given it has been delightful.

Before I discuss the slow roll off setting, let’s instantiate what the S2 Digital nails. I measure THD distortion at 0,0008% in unloaded signals on an ADC system that can measure down to 0,0002%. Stereo crosstalk tops out near -110dB, and both jitter and IMD are ridiculously well controlled- both for the S2’s price and for a bus-powered USB device. By and large, the S2 is load agnostic. Unloaded or no, it keeps its basic frequency response controlled down to a maximum difference of roughly 0,5%. That despite driving an Earsonics SM2, an Audio Technica ES7, or a Beyerdynamic DT880. Of the three, the SM2 stresses it the most, amping absolute THD by 75x at maximum volume (0dB), which may sound extreme, but which ends up at 0,063%, or completely inaudible. It controls IMD at the same volume to 0,1%, which is also inaudible, and represents a jump of 11x from base, unloaded results. Loaded results mimic those of the 3000$ Sony NW-WM1Z, while unloaded THD and IMD and stereo crosstalk scores favour the S2 Digital.

I’m really surprised at how well the S2 Digital drives the ES7 at maximum volume. THD ticks upward from 0,0008% to 0,0009%, and IMD from 0,0089% to 0,020%, and the ES7 is pretty tough to cleanly drive at maximum volume. Speaking of, through my iMac’s USB, the S2 Digital gets about as loud as an iPhone 6, which is enough to drive many headphones, but won’t trounce high-current planars. Through better USB implementations, the S2 can attain higher decibel levels. Coax and optical input results mete out similar results.

Overall, the S2 Digital’s performance is good to great, but not quite top shelf. What is top shelf is its handling of MQA and DSD, not to mention Red Book files. Obviously, both MQA and high sampling rate DSD files require feeding the S2 Digital via USB. Filters work any and all file types.  Despite problems in my iMac’s USB implementation, I still prefer listening through it precisely because I can take advantage of everything from MQA to the S2’s remote control.

Let’s talk filters. The slow roll off begins its high-range descent at 5K, dropping by roughly 5dB by 18kHz, viz., it’s a minor roll off certain not to perturb purists. It retains a good amount of high end detail, but rules out the harshest treble spikes. Simultaneously it evens out possible stereo drops under load. All frequencies retain roughly equal stereo sound pressure, which make the S2’s soundstage deep and rich. The total effect smooths out many of the harsh upper-end cues which annoy me about the DT880/600, and which make Susvara sound even richer.

Naturally, the S2 Digital is pushing its limits through both headphones. For my preferences, it nails the DT880, but under extremely dynamic passages in both space and classical music, Susvara needs more voltage. For most popular music, however, the S2 powers that headphone well, just not to loudness extremes. Because it keeps IMD low, even at the loudest volumes you’ll never hear your favourite tunes sheer or clip.

And whichever filter you chose stays active even after powering down. The other filters exact different final opinions, but the S2’s basic tone: rich, tending to midrange warmth, and Z-axis deep stages, change only just.

Before I file out of this section, let’s talk noise: my home has a ground loop. Fixing it is a bugger. Some DACs and amps are more susceptible to it than others. Unattended on the desk, the S2 Digital makes slight shhhhh sounds in the background, but when properly grounded, is dead silent. This goes for hardware hiss as well. It’s as free of hiss as Antelope Audio’s high end Zodiac systems, and on par with some of the best smartphones and DAPs. All of the above satisfy head-fi’s self-declared hiss king.

Because the S2 Digital is rich across the mids and stereo deep, reference-neutral to bright headphones will resolve the greatest accents its of which its filter system is capable. But you can really deep dive with warmer headphones. My favourite combo? Maybe the Grado GH-2, whose stereo depth isn’t in the same league as the DT880 or Susvara. The S2 really focuses on that headphone’s strengths.

End words

The S2 Digital’s filter system should satisfy everyone but the hardcore valve listener. The S2’s got a solid, and largely load agnostic output system. Everything from its remote control to its input array work brilliantly with 99% of upstream sources. It syphons powers via USB and outputs decent power to most headphones. Its rich tones and encompassing stereo output are nearly in a league of their own in the camp of devices that ride on ESS DACs.

It does all this at 349 Euro. In my books, it is a coup.

Well done.


Back before he became the main photographer for bunches of audio magazines and stuff, Nathan was fiddling with pretty cool audio gear all day long at TouchMyApps. He loves Depeche Mode, trance, colonial hip-hop, and raisins. Sometimes, he gets to listening. Sometimes, he gets to shooting. Usually he's got a smile on his face. Always, he's got a whisky in his prehensile grip.


  • Reply February 19, 2018


    Great review! The multiple inputs, remote control, display, and ability to play MQA really have my attention. Wouldn’t be using the amplifier section though and wonder if I would be wasting money on it.

    Currently use a Schiit Modi Uber for my speaker system and will sometimes hook up my iFi Nano Black Label as well for MQA listening. I’m considering an upgrade to a dac with more inputs that also plays MQA. For the same price, I could buy the non-multibit Bifrost and possibly upgrade in the future.

    Curious which I would get better sound with.

    Pro-Ject Pre S2 with MQA or Schiit Bifrost?

  • Reply April 21, 2018

    Phil Simpson

    Hi, great review. I was interested in the image of the Pre Box S2 and Apple Super Drive and was wondering if it’s possible to use the Super Drive dirctly as CD transport and control it (play, pause, skip etc) from the Pre Box S2’s remote.

  • Reply June 5, 2018

    Paul Donovan

    Hi, I received mine today. The headphone connection is terrible. You have to get it in exactly the right spot or you will be hearing distortion. I do not have this problem with other devices. I think it is the way it is manufactured but I am going to send it back and get another one. Sounds very good when the connection is working.

  • Reply September 27, 2019

    Steven Sylvester

    I was so very impressed by your reviews on the following three Headphone amps. since reading them I have tried very hard to pick a winner from the 3 contenders??


    I am a novice when it comes to audio and prone to wasting money through lack of knowledge you guys and contributors would be really helping if you can PICK THE NUMBER 1 FOR ME??

    I have never owned a real Hifi,let alone a headamp and Headphones,I have 3TB of music at 24\96 and upwards with some DSD files e.t.c. this music is held on SEAGATE EXTERNAL DRIVES played through a LENOVO IDEAPAD which only has USB Normal size + 1 x C-USB +headphone socket. I have no way to audition any of the above so expert advice is crucial and will be very much appreciated.

    Regards Steve

    PS I hate all FORUMS!!

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.