Unfortunately I didn’t have them both - U1 and S1 - at the same time, but I believe the S1 sounded better than the U1/Directstream DAC. I could be mistaken because ear witness testimony is unreliable with the passage of time.
I sold the S1 because I reasoned I could get a lower price Lumin and use my Directstream DAC and have as good or better sound than the S1and recoup some money I spent.
At at the end of the day I’m happy with the U1 and Directstream DAC...but that S1 put a spell on me...