Does the M12 have a normalize function?
OK, it does not (the term doesn't exist in the UM).
I'm gonna discuss some things about my workflow that I hope will be helpful.
I took a look at the block diagram and the level diagram, and unless I'm missing something, it appears that there is 18dBU of headroom in the DAC and that the DSP engine can function up to 0 dBFS. Given that the mic/line inputs have lots of gain (on the order of 50/60 dB) I don't see why anyone would have trouble getting their levels up to a reasonable recording level.
Given the dynamic nature of performed music like drums and guitars, the peak levels might prevent one from going really hot into the recorder, but, as long as there is a reasonable level and the noise floor is low, the digital audio level can be manipulated to wherever it needs to be, later.
I think the issue might be monitoring levels rather than mixing levels. FWIW, my target mixdown level is -20 dBFS. That gives me the headroom that I need for whatever peaks are happening in the mix. Things like drums and acoustic guitars can be as dynamic as they need and I don't have to worry about clipping (over 0 dBFS). Once I have a stereo mixdown file, I then proceed to do the Mastering to get the track level wherever it needs to be for delivery to whatever platform. These days I'm using the Adam T8V powered monitors and they have plenty of gain. I also use a Mackie Big Knob Studio, and that has trim pots on it to match levels between sets of monitors. I have a set of Tascam VL-S3BT monitors that I really like a lot. I have those matched to the apparent volume of the Adams by using the trim pot on the Big Knob Studio. My point is, I have a lot of gain and precise control over my monitoring. The importance of that cannot be overlooked. Some of the comments from Model series users seem to indicate that studio monitoring is of secondary importance to the recording. Monitoring is everything. You cannot mix what you cannot properly hear.
During mixing, the level at which I have to crank up the monitor volume is considerably higher than when Mastering and as I raise the level of the limiting, I turn the monitors down to compensate for the increasing level of the track.
So, maybe some of you Model series users need to turn up your monitor level. It really shouldn't matter what the mix level is in order for you to properly hear what you are mixing. If I'm working with something very quiet, I turn up the monitors and carefully do what needs to be done.
The Model 12 does not have a dedicated Control Room monitoring section. The UM suggests using the Sub bus and fader for monitoring. Looking at the block diagram, that is less than ideal because it's pre the final EQ section. I would probably try using the Main outs and a monitor controller like the one I mentioned. That way, I would be monitoring off the same bus that's going to the mixdown track in the recorder.