I am using Pre Box S2 Digital for two months. Today I had a chance to evaluate sound quality switching it to 5V power supply from iFi Audio iPower. Before Pre Box was fed from JCAT UCB card which is powered by Sbooster botw p&p eco linear psu. Challenge was to see - if external power supply could do better than ultra clean USB power. As a reference I used 24/192 Robert Ludwig's MasterDisk (Dire Straits) and several DSD files (Diana Krall, Sting, Mike Oldfield).
Summary - there is noticeable improvement in sound quality when quality external power supply is connected to Pre Box S2 Digital DAC. I should admit- it is not big improvement since my dedicated USB card has already clean, noise-less power supply. Sound difference can be described as:
1) more presence - vocal and several instruments moved to me a bit closer in the soundstage (quite a pleasing feeling);
2) muted background - it become more silent increasing clarity of instruments and vocal sounds;
3) precise speed - each note, drums hit had better attack, sharpness (but not sounding harsh), and bass notes were more accurate.
Even current power change made my system sound just 2% better, I like such upgrade.