When using the iriver with rockbox, the gain you set in dB is the setting of the audio chip, I think the rockbox wiki has some schematic of the audio circuit if you're interested. The ST-9100 has no scale so no idea there...
What's the importance about knowing unity gain? I set my iriver to 10 or 15dB, adjust the ST-9100 to get good levels and leave the rest to the agc-safety code

If you're really concerned, you can leave the iriver at 0dB and only use the ST-9100 gain setting and go manual....