Does the Impact Twin output 16 Bit encoded audio files if requested by the DAW or will they be dithered down from 24 Bit?
From the DAW to SPDIF-/ADAT-out there is no dithering so what you do in the DAW goes out directly.
Any 16 bit dithering should then be done in the DAW and if you like to listen to 24 bit resolution instead, you can do it directly by running the DAW and files in 24 bit and dithering OFF or to 24 bit. You might want to have 2 DAW buses with the master mix output (for example done via a send from the main output) – one with 16 bit dithering and one with 24 bit and then A/B that in the Impact Twin to hear any differences.
The same goes for SPDIF-/ADAT-in to DAW signals. No extra and hidden handling by the Impact Twin – just perfect audio transmitted. What you record is what you get – DAW set to either 16 or 24 bit will decide that.