I use a similar technique to set levels of my monitors for mixing, but this is a bit different.
 
In essence, I am sending 16 channels out of my PC into 16 channels in the mixer. This signal will get summed in the mixer and then sent out the mixer into two tracks coming back INTO the PC. There are no detents on the mic-pre volume knobs nor on the faders, so simply setting a knob or fader to unity based on looking at it won't be precise enough.
 
We want them all to be at unity so there is no skewing the results by having some tracks slightly different in volume. Also, using this technique ensures that if I have to move something in the future, I can always re calibrate and reference old mixes (all 2 of them! LOL).

backwoods
From Sonoris website:
 
Calibration is simple, first select what channel to use for the calibrated pink noise (bandlimited 20Hz-20KHz or 500Hz-2KHz), L, R or both. You can calibrate your monitors at this level to show 83dB on a slow, C-weighted SPL meter using your monitor control. In stereo mode add 3dB. Mark the monitor control levels for each scale and you are done!
 
The 500Hz-2KHz noise signal can give a better and more stable reading on the SPL meter under some circumstances. Please try both options and see what works best for you.

We are not calibrating SPL levels for monitors here. We are calibrating precise levels within a DAW. These two things are very different.
 
On that though tones are not great for calibrating SPL monitor levels either. You only have to move the measurement microphone a few inches and the level can change by many dB. Band limited pink noise is a good source for this job. Except you have to adjust the gain of band limited pink noise after you apply the filtering to full range pink noise. That is very important.

The insert points are the best place to insert a signal that completely bypasses the whole front end of that mixer.  The inserts will probably be TRS.  Which means the tip carries the signal coming from the preamps and the ring is the way into the signal path for the channel.  So if you push a plug right in it won't work.  But like Mackie mixers you may be able to insert the jack part of the way in order to get to the ring connection. 
 
Or you wire a TRS lead and only connect to the ring connection. The insert points are not balanced either.
 
The specs don't actually mention the insert receive nominal level but for other line inputs it is 0 dB VU (which is not +4 dbU either. It means 0.775 v rms not 1.23 v rms)
 
Your interface might send out +4dbu for a certain digital ref level. That is what I would find out first.  Check your interface specs. What is the max output level at 0 dB FS.  If it is say +22dBu then in order to get +4 dBu out of your interface you would have to work down at -18 as being the ref level.
 
It would be OK to send +4 into the insert points because the headroom is enormous e.g. 20 dB or more so it would not be an issue driving the channels little harder.

Hi Jeff,
 
I just plugged the TRS connections from my DB25 snake into the inserts and they all work perfectly! I get what you are describing, but they are all working in my case...
 
The UAD let's be control the the reference level of all pairs on the Apollos by choosing either +4 or -10. Right now they are all defaulted at +4 dBu. 
 
The signal is good and strong coming into the board and in fact now that I've "roughed in the levels" it's grooving and I'm resisting the urge to create a copy of the project and wander off with it. Something sounds great, but at this point I may just have the drums and bass too loud or something; so I'll stay focused on calibrating.
 
I had an interesting experience. In Ableton all tracks are stereo tracks. So ran the bass out in stereo to a pair of stereo ins and it all sounded fine. For giggles, I thought I'd try the bass in mono so I put a mono plugin on the track that goes out to the Midas, switched to send on only one out and not a stereo pair, and chose a different single insert on the board that was a stereo track.
 
Now the bass was too low. I think maybe the bass was mono to begin with and when I put the mono plugin in it I halved the volume. I need to test. Anyway, I put a gain switch on at +6db and it sounded good. I cranked it to +9 db and it sounded GREAT! Not objective by any means, but still a shload of fun! :)

Ive done a of stuff ITB but summing OTB works best for me. I have the D Box and I really really dig it. So much so that Im looking to buy a 2Bus LT in the future to take all 24 out of the box.
 
Sum goes into a Langevin DVC and then into a Daking FET3 then back into the DVC Limiter section and then back into the DAW.
 
Don't have to use it all with all the in/out switches but its there. I dig it.

OK, here's a first set of files. :)
 
1. BelieveITB.wav is the In the Box version. This is how the song was originally mixed and rendered.
 
https://www.dropbox.com/s/9xkvbs6kp1chhrc/BelieveITB.wav?dl=0
 
2. Believe direct no eq.wav. This is the first attempt at summing. All tracks where tested for unity with a 1k sine wave. VUMT wasn't playing nice so I used Dorrough. That's not quite exact though, so I may want to do it with VUMT again. At any rate, they are close!
 
https://www.dropbox.com/s/50s24hrolsdpxxo/Believe%20direct%20no%20eq.wav?dl=0
 
There is no limiter on these, so feel free to use your own.
 
I added this to a project, solod one and played the other then mapped the solo button for each track to the same midi key on my Novation LaucnControl. This allows me to toggle between then with one touch of a button.
 
I absolutely hear a difference! I also don't ALWAYS hear the difference! :)
 
Tempo is set to 88, so use that for a reference with my notes:
 
PS: I believe they are level matched but if you find they aren't, please let me know.
 
1.   The sound effects at the very beginning sound a bit more distant in the outboard mix. The kit almost seems a bit lower.
2. But at measure 5 (10s) the kit sounds pretty much identical.
 
3. At measure 19 the ITB mix guitars just disappear. I can't find any differences in the mix though to explain this. :/
 
4. The Aux buses with the reverbs are much more pronounced in the outboard summing mix. They would need to be brought done. You can hear them when the vocals start at 29. 
 
5. The build up/rise that starts about 42 sounds "smoother" to me on the outboard summing. 
 
6. The bass line that starts around 51 (one of my favorite parts, thanks Bapu!) seems more focused in the out of the box mix and seems to sit better to me.  The low end in general is much better and very noticeable here!
 
7. The synth bass line that starts about measure 64 again seems more focused and better blended in the out of the box summing mix.
 
8. At measure 70, the background "ooohs" seem fuller in the out of the box mix. 
 
Let me know what you hear! Also, let me know if you think I messed up the methodology at all...
 
This is actually challenging, but a whole lot of fun! I've got some other outboard gear on order and will compare methodologically as well when it gets here. :)

Dickie Fredericks
Ive done a of stuff ITB but summing OTB works best for me. I have the D Box and I really really dig it. So much so that Im looking to buy a 2Bus LT in the future to take all 24 out of the box.
 
Sum goes into a Langevin DVC and then into a Daking FET3 then back into the DVC Limiter section and then back into the DAW.
 
Don't have to use it all with all the in/out switches but its there. I dig it.

Dickie,
Just my initial tests tells me there is <something> different, and from what I can tell slightly better (especially in the low-end). I've got more to test, but check out the files I just posted if you get a chance and let me know what you think...

OK listened to these.  Firstly they don't null in any form.  Not even close.  The two waveforms are very different.  So that  means the test is not accurate.  When Audio Technology did this they got very close to a perfect match of waveforms.  Something that is not happening here.
 
I think the mixes between the two are too different.  I am hearing more kick in one etc..
 
Also has the ITB mix got no eq etc everywhere the same as the OTB mix?
 
My impressions are there is nothing I could not do in the ITB mix that is happening in the OTB mix.  i.e. not worth the hassle of sending everything out to mixer etc...
 
If you read that link I posted right at the start it is saying someone who does a better ITB mix will sound better than someone doing an ordinary mix OTB. I agree with that.

They are very different - totally agreed.
 
However, all I did was route the outs to the Midas and sum there. I used 1k sine wave to get the midas to have all returns at unity. That's it!
 
So yeah, the are very different. Cool if you could match the differences. Not sure I could, or would know what to look for. However, being able to build off this base? Yeah, that is cool to me.
 
So, where could my methodology have gone wrong?

Yes it is tricky getting an identical mix with ITB and OTB setups. I think you need totally mono tracks. They should only be panned L C and R. The pan law for your mixer needs to match that of the ITB version. You cannot change the pan laws for the mixer so you need to find out what it is.  (i.e. send a tone from a channel to one side of the Midas mixer e.g. L. Set the output meter to read 0 dB VU on the left channel. Then without changing anything, pan centre and see how far it drops down. It will either be 0,-3, -4.5 or -6 db. Then you need to set the ITB DAW for the same pan law.)
 
I think the best way is to set the mix via clip gains only and leave all your ITB faders at unity. This is hard in Sonar because you cannot do it easily.  In Studio One you can by just grabbing the gain handle of the audio event on each track and pulling it up or down. The level and the waveform height change. 
 
You also send all the tracks out to the mixer and set all their gains at unity too. (which you have calibrated of course to all be identical) 
 
No plugins anywhere ITB of course and all EQ's bypassed on the mixer. No dynamics or reverbs anywhere.
 
Theoretically the same mix should exist ITB and at the stereo outs of the mixer which of course you record back into your DAW.
 
I bet if you did this, things would be much closer.