I'm either running
Oktava mk012 > PS2 > AD20 > D8
or OKM II/R > A3 > AD20 > D8 (for stealth recordings).
The A3 (battery module) has a -20dB pad so, with the minimum +17dB of the AD20, that makes a +3dB at minimum gain.
Since I posted my initial question, I recorded one show and found out that with the knobs of the AD20 placed at "0", my levels on the D8 were just hitting -12dB.
Therefore, I didn't have to worry about the gain knobs position.
Unfortunately, the recording is not great... very muddy and distant...
The lightness and presence that I hear with the oktava are not present at all with the OKM.
I also experienced some "distorsion" with the Oktava mk012 > PS2 > AD20 > D8 setup.
I'm not sure if this is due to the DAT or to the AD20... since this is not clipping but distorsion, I assume that the signal passed to the DAT (by the coax out of the AD20) was too hot (?).
Anyway, I now have Shure attenuators to prevent this problem.