Sidssp,
Am I reading these right?
The SMS-1 input can be overloaded into distortion. To prevent this, adjust the output level of the source appropriately. Done. No issue (at the preferred -6db input level, distortion for both the Behringer and the SMS is inaudibly low - per those graphs). Granted, the Behringer measures a bit better, but - given these very low THD levels at these very low frequencies - I really believe that you're looking at hairs on the arms of the angels on the head of the pin.
The LF roll off shown for the SMS-1 is down app 2db @20hz. I filter at 25hz anyway for my music only system. This might be an issue for HT use, but not IMO for music.
Unless those results are viewed in light of HT use, they will be awfully misleading. Unless I'm missing something -and it wouldn't be the first time for that!
Marty