On a whim, I picked up the "Stream Box S2 Ultra," got an offer I couldn't refuse
I've seen this advertised in the Pro-ject materials as the "head unit" to control the DAC all over the place. Then I started looking at John Westlake's plans for the MDAC2, and I realized that the Stream Box S2 Ultra + Pre Box S2 Digital are basically an unbalanced version of that unreleased project. So, now I know why there is no power button with the Pre Box S2 Digital - when its plugged into the DAC port of the Stream box S2 ultra, the S2 ultra powers it on and off. While they are sold seperately, I'm fairly sure they were designed as a pair.
On a side note, it may be placebo, but to me it seems like the USB Detox in the Stream Box S2 ultra improved the signal more than the Pre Box S2 digital by itself