@Jerryfreak, thanks for all the work you've done! From what's been answered by Zoom I get that the digital trim also in 24 can only be adjusted after the analog input stage.
It seems, the, that the padded line-in input may produce a bit more noise as a result; I still wonder if in real world use the unpadded +4 db limit may actually cause overload problems/distorted file even in 24 bit mode. Given the F6 design, maybe something more than +4 would have be a safer option?
If I'm right, recording two parallel tracks in 32 and 24 will result in having a 24 bit files probably ok "out of the box" if you've trimmed levels ok, and in a 32 bit files which free from setting inaccuracy; both files, though, are equally subjected to the same risk of input overload - if rare, I argue.
I just wonder if the "traditional" F4/F8 of recording a "more generous" file and a safety track 12 db lower could lead to a safer result and less fiddling at the end of the day.
Apart from measurements, which are interesting indeed, I'd like to figure out what performance F6 will give in an average scenario - i.e. recording a classical or jazz concert or a choir or whatever - with a pair or average sensitivity mics. How does it compare in sound quality to an F4/8 or Mixpre or even a cheaper recorder like Zoom H6? Reading here and there it seems that, using condenser mics, the limit in noise floor is most likely stated by the mic self noise more than preamps.