(Internet mis information here I think) I found the opposite to be true especially when you start dealing with 4 DAW's.  Level setting to 3 decimal places should be avoided in my opinion.  They might all handle -7.97 in a slightly different way.  Where as -8 is going to be much easier and happier for all of them.  You can get a great mix with raw tracks using whole numbers for level settings. e.g. -7 dB or -8 dB or -9 dB etc...1 dB changes in level will be very subtle in terms of volume levels. 

 
I think if it's still not nulling with all dithering turned off, and the null is only ruined when you move a fader, that fader move has to be the clue. The only thing I can think of is that moving a fader by a numerical amount in S1 delivers a volume difference that is slightly different than moving Sonar's fader by exactly the same numerical amount. 
 
Until you can dismiss that possibility, you cannot reach any conclusion about the summing engine. 
 
If the same fader move in both DAW's results in a slightly different volume level for the track in question, then they won't null. The frequencies in the summed output will be different in each DAW. If that's the case, then it's more likely that both DAW's sum in exactly the same way, but their faders behave slightly differently.

In SONAR Platinum I found the Pan law settings in Preferences> Driver Settings. Mine is by default set to "0db center, sin/cos taper, constant power"

I found out how to change the pan law in Studio One. You have to use a plugin...ha ha ha. It is called Dual Pan. I placed it on the "Main" and set it to the same as SONAR which is 0dB center.

 May be there is still some difference due to sin/cos taper, constant power 

Hey guys, you aren't going to believe this, but I found a way to make Test 3 null!!!!!!!!!!!!!!!!!!! I can't believe it. Yes, you heard correctly. I was able to get SONAR and Studio One to null on the test 3 with the kick at 0, snare at -8, and overheads at -13.
 
Here is how I did it...
 
1. As suggested by azslow3, I manually typed in the values in both SONAR and Studio One.
 
2. I changed the Pan Law in SONAR to match the Pan Law in Studio One. (Apparently using the plugin to change the Pan Law in Studio One doesn't work.) That begs the question: Is there a real way to change the Pan Law in Studio One? I would like to test changing the Pan Law in Studio One to match that of SONAR to confirm that a null will work that way too. Can someone suggest a reliable way to change the Pan Law in Studio One that actually works?
 
3. Removed all dithering from both DAWs (I am not sure that this would have really mattered either way.)
 
So, my conclusions...
 
sonarman1 was #$@& right! It did have something to do with the Pan Law, even though panning was not being used. Something in the way each DAW handles stereo tracks or something... I didn't figure this out right away because when I tried sonarman1's suggestion about Pan Law, I tried to change Studio One's Pan Law, which seems to be very difficult to do. I hope it is possible though. I still want to confirm that these two monster DAWs can null going the other way. But changing Pan Law in SONAR is easy, so I tried it...and it worked!!!!
 
Also, slight variations do matter as azslow3 pointed out. Typing in exact values I feel made a difference in getting a null. 
 
Jeff Evans is also right. However, due to default settings inherent to each DAW, out of the box, there will be differences in sound. It is a bit of a job, but if you configure all settings appropriately, these DAWs will indeed pass the null test. Jeff was right about that. Studio One does make it a bit difficult to change some settings. I wish it were a bit easier.
 
So in conclusion, out of the box, there are sound differences, but that is due to some default settings. If configured exactly the same, the will put out exactly the same audio result, as Jeff Evans pointed out.
 
So, as I eat a little humble pie... Jeff, please accept my apology. I was so passionate about this and had a driving need to see it through. I really wanted the results to null from the beginning and it was really bothering me that I could not get them to null. I had spent my money on a product (Studio One) and wanted to feel like I was getting just as quality of a tool as I had in SONAR. The fact that I could not get a null was gnawing at me. I wanted to see the hard evidence, the science if you will, behind it and know that it was so. I wasn't getting that so I had to keep pushing. I am so glad I did because now I feel a whole lot better about Studio One.
 
I still would like to see some improvements in some areas of course, but that is for another thread in another place. This was the big issue I needed to get past.
 
A thank you to sonarman1, azslow3, and Jeff Evans on shaping this journey and seeing it through.
 
Anyone out there considering Studio One and wondering if it can null with SONAR, it can. I have made sure to satisfy that question.
 
Edit: I would also like to thank CW3948368110 and sharke. Your help was also invaluable in this. Thank you.
 

+1 on Azslow's comment.

Sylvan
Hey guys, you aren't going to believe this, but I found a way to make Test 3 null!!!!!!!!!!!!!!!!!!! I can't believe it. Yes, you heard correctly. I was able to get SONAR and Studio One to null on the test 3 with the kick at 0, snare at -8, and overheads at -13.
 
Here is how I did it...
 
1. As suggested by azslow3, I manually typed in the values in both SONAR and Studio One.
 
2. I changed the Pan Law in SONAR to match the Pan Law in Studio One. (Apparently using the plugin to change the Pan Law in Studio One doesn't work.) That begs the question: Is there a real way to change the Pan Law in Studio One? I would like to test changing the Pan Law in Studio One to match that of SONAR to confirm that a null will work that way too. Can someone suggest a reliable way to change the Pan Law in Studio One that actually works?
 
3. Removed all dithering from both DAWs (I am not sure that this would have really mattered either way.)
 
So, my conclusions...
 
sonarman1 was #$@& right! It did have something to do with the Pan Law, even though panning was not being used. Something in the way each DAW handles stereo tracks or something... I didn't figure this out right away because when I tried sonarman1's suggestion about Pan Law, I tried to change Studio One's Pan Law, which seems to be very difficult to do. I hope it is possible though. I still want to confirm that these two monster DAWs can null going the other way. But changing Pan Law in SONAR is easy, so I tried it...and it worked!!!!
 
Also, slight variations do matter as azslow3 pointed out. Typing in exact values I feel made a difference in getting a null. 
 
Jeff Evans is also right. However, due to default settings inherent to each DAW, out of the box, there will be differences in sound. It is a bit of a job, but if you configure all settings appropriately, these DAWs will indeed pass the null test. Jeff was right about that. Studio One does make it a bit difficult to change some settings. I wish it were a bit easier.
 
So in conclusion, out of the box, there are sound differences, but that is due to some default settings. If configured exactly the same, the will put out exactly the same audio result, as Jeff Evans pointed out.
 
So, as I eat a little humble pie... Jeff, please accept my apology. I was so passionate about this and had a driving need to see it through. I really wanted the results to null from the beginning and it was really bothering me that I could not get them to null. I had spent my money on a product (Studio One) and wanted to feel like I was getting just as quality of a tool as I had in SONAR. The fact that I could not get a null was gnawing at me. I wanted to see the hard evidence, the science if you will, behind it and know that it was so. I wasn't getting that so I had to keep pushing. I am so glad I did because now I feel a whole lot better about Studio One.
 
I still would like to see some improvements in some areas of course, but that is for another thread in another place. This was the big issue I needed to get past.
 
A thank you to sonarman1, azslow3, and Jeff Evans on shaping this journey and seeing it through.
 
Anyone out there considering Studio
One and wondering if it can null with SONAR, it can. I have made sure to satisfy that question.
 

A great exercise! Everyone learned a little about S1P And perhaps a bit about DAWs in general

And all said "this is a good thing"!

I forgot to add a comment about dithering...
 
It the test we are speaking about, the different between test files is audible. I hope it is clear that since we are speaking about 24bit files, to perceive any difference from algorithms (even without dithering vs Pow-r 3) , we need some tool capable to show signals under -140dB. Or amplify it at least +40dB (till someone participating in this discussions is not a human).  

Look I am just happy that you have got a different result.  And the right one too!  I think if it just comes down to pan law settings and things then we can all agree you can make a pretty decent recording in any DAW.  And as we are not nulling DAW's often, after all we are only working with one usually, you will be setting panning and level by ear.
 
Sylvan you can certainly change pan laws settings.  Use the Dual Pan Plug In and over on the left under the main heading is a menu. Click on it and all settings will show up. Select it there. I have found the default -3dB centre is pretty nice for everything.  I don't change that very often because I don't feel the need to.  I find panning sounds more obvious to me in Studio One though after coming from Producer 8.5.  
 
I have enjoyed this thread because it has got me thinking about doing a null test with my digital mixer summing stuff instead of summing everything ITB. I am curious with this for sure.  There is something going on there. 
 
Null testing is also very cool and can be used in other ways too.  Like setting up a compressor.  As you know we have to often set up compressors over individual tracks or mixes. You can set up a parallel path and phase inversion so you only hear sound when the compressor acts on the  signal. When it is not doing anything you will hear silence.  So after setting all this up, you switch all that off and set up your compressor with audio present as per  normal.  But when you switch into the silent mode and only hearing it acting then, if you hear audio coming in and out in a very jerky manner then it means you can set your  compressor better.  Often by just refining your initial settings, you will get much snootier transitions as to how the sound comes in and goes away.  Once you get that nice, when you switch back to normal audio plus compressor operation, you will hear much nicer compression.  It will sound very musical and smooth and locked into the music much more.  More transparent too which we often want. 

Wow! I was just about to post that setting up dual pan plugin in S1 masterbus is indeed doing nothing. (May be I am not not doing it properly?). But seriously how else to do it?

Great job Sylvan now its obvious there was a solid reason behind why many including me was wondering about the difference in sound.
It seems with sonars default pan law 0db constant your mono track is gonna sound +3db if panned to center. 
So thats why you felt your low end was better in Sonar coz your kick was in mono and +3db in level in comparison to S1 with -3db pan law.
 http://forumsarchive.presonus.com/posts/list/0/37072.page this post is useful in understanding pan law.

Jeff Evans
 
Sylvan you can certainly change pan laws settings.  Use the Dual Pan Plug In and over on the left under the main heading is a menu. Click on it and all settings will show up. Select it there. I have found the default -3dB centre is pretty nice for everything.  I don't change that very often because I don't feel the need to.  I find panning sounds more obvious to me in Studio One though after coming from Producer 8.5.  
 

I saw that plugin and tried to use it, but it didn't seem to have any effect or change. Maybe I am using it wrong or something...