The passthrough is exactly what it claims to be - the analog signal from the RCA inputs bypasses the THX amplification stage
and the volume control. See some info from the Q&A section on the product page linked by
@RickSanchez :
Headphones: SU-9 BAL out → [XLR] → Monolith BAL in → headphone / volume controlled by the 887
Active speakers: SU-9 SE out → [RCA] → Monolith SE in → Monolith PASS out → Speaker RCA in / volume controlled in software or in the SU9 (will impact the headphone path too) or at the speaker if possible/desired.
Both paths will be active at all times, so you'll need to remember to turn the digital volume down when switching to speakers.
I guess it's clear now why so many on ASR are skeptical about the SINAD race and would prefer DACs with better feature set instead