AFAIK DC does not supply passive attenuation specifications for the DC ONE X. Here is the data sheet, FWIW:
https://www.flytheone.com/docs/DC-ONE-XP-Data-Sheet.pdf
DC is a great company. You might give them a call, or drop them a line, because they might have an answer for you!
As for how well the DC ONE X will work in a passive mode if the ANR fails (dead battery, etc.), I can only tell you it is an annoyance, not an emergency, in an R44. It certainly sounds like the environment is more challenging in an S92, but on the other hand the A-weighted SPL in a 44 at idle is about 80dB and over 90dB at full power. I fly a ship with air conditioning and it adds its own noise. Still not an emergency. Here is some cockpit sound measurement data that you might be able to draw upon for comparison purposes:
https://bib.irb.hr/datoteka/438054.in09_944.pdf