MENU > INPUT > TRIM LINK
See the manual, pg, 101.
Your answer got me thinking, and I am away from my decks and rusty with my knowledge right now, but if the knobs on the F6 are not gain, but fader, and gain is set automatically, does having those faders not balanced with each other affect the actual recorded tracks? On the one hand no, nor does it matter because you can balance in post, but does it affect the actual recorded gain levels used by the recorder if the faders are not balanced?
I know we tend to still think of those knobs as affecting gain, but that is not the case even if it feels like it does.
Even though
gain may be fixed,
recording levels are not. Depending on your recording mode (linear vs. float), those knobs could be adjusting individual channel input trims (post-ADC), the individual ISO track recorded levels sent to the SD card, or acting as faders for the stereo downmix. (I'm differentiating between "recorded levels" and "faders" here, because faders are the last adjustment you make before going out to your main mix.)
Check out the block diagrams in the manual on pp. 193-198. I always use 32-bit float mode, and I have my knobs set to adjust "Rec Level" as described on pg. 194. Keeping them at the default "Reference Level" is useless to me, because I never use the stereo downmix, and this way at least I could adjust ISO track levels if I wanted to. Of course, in 32-bit float mode with the multi-ADC setup this recorder uses, adjusting levels isn't really necessary, but I like to watch the meters dance.
So, what all of that means is that you will absolutely be recording different levels on your individual channels if your individual track knobs are set differently. That would be true no matter what mode the recorder is set up in. This is why I have my input trims all linked in pairs.