I guess I keep reading these threads with posts and frustrations about channel linking, setting up the Mixpre with custom mode using basic gain, etc., and feel like I should try to make a plea for people to re-think their old work flow.
I have recorded 25-30 sets now with the Mixpre-6 in a number of different venues, with a large variety of types of music (from acoustic duets to more louder jam-grass to more typical rock sets), using different sets of mics. I've never used anything but advanced mode, and I've always pre-set my gain before the show started and never changed the gain settings during a set. Even using a couple sets of new-to-me mics in venues I've never used them at, every set I've recorded has ended with peaks in the range of -18dbFS for the quietest recording to -7dbFS for the loudest recording.
Bottom line, I don't think it is very hard with our level of experience as tapers to guess beforehand approximately how much gain you need. And the Mixpre series is incredibly quiet, even my acoustic stuff that ended up at -20dbFS shows no hint of noise once I've boosted/normalized the recording in post. Short of incredibly quiet studios or nature recording, for concert recording in almost any setting we can imagine, if you ended up with way too little gain and a recording that topped out at -40dbFS, you won't notice any noise hit once you normalize the levels up in post, at least based on SD's published specs. This seems to have been borne out when the earlier firmware resulted in recordings at -40db or -45db if you had turned off the LR mix tracks in the set up (now fixed). People who had that problem reported no issues once they boosted/normalized their -40db recordings up to 0dbFS.
I'd really recommend folks trying to record with this new thought process/workflow. If you set up gain beforehand and don't touch it, there's no need for to get into the custom mode with basic gain, no worries about channel linking or 4-ch linking since they will be "linked" at the same gain up front. The Mixpre series allows you to set a precise level of gain on all channels, if for example you're using a 4ch ambisonic mic and set up and match the gain beforehand and never change gain during the set, there is no need for channel linking.
I guess I'm just missing the reasons people have for wanting to control gain during the set, but with incredibly quiet preamps (noise at -130dbV) and an unbelievably wide dynamic range (120db), the Mixpre can be easily used with gain set very low and conservative (leading to very low recording levels) and then boosted in post with no discernible penalties. Guessing what gain levels you need beforehand when you are just shooting for peaks at -30dbFS (but willing to accept -1dbFS) leaves a wide target and doesn't require much knowledge and experience to achieve.
If people are willing to try this as a new workflow, it sure seems to avoid many of the complaints folks have about this recorder. That's my long-winded public service plea. Sorry, not trying to lecture experienced tapers on how to approach their recordings, just trying to emphasize that new technologies and vastly improved recorders and preamps allow for new ways of doing things which didn't work before. (Compare for example the recording I made years ago with a Sony D7 DAT using phantom power supplies and the internal D7 preamps. Peaks at -16db, oh god the noise on that recording, sounded like listening to music on a boom box by the ocean.)