This is all too confusing - I just went and bought a Midas Venice 320 this morning and ordered a UAD Apollo 16. 
 
Time for some analog summing!
 
Who wants sound clips?! (If so, Ed needs to fly to Nebraska...) :)

Well I have just done null test with the multi track drum session that my son played for the Cirque audition. I imported the (pristine) tracks into both Studio One and Mixbus 32C. No effects used anywhere on either. No saturation used in Mixbus either. Only relying on the so called Mixbus summing sound magic to set them apart.
 
Most the tracks actually could stay at unity gain. I just increased the snare and kick by 3 db and lowered the OHeads by 6 db. I panned the OHeads L and R and panned the three toms L C and R. All else stayed centre.
 
Interesting that I got no discernible difference in sound from either. Virtually a perfect null as well. So I agree the video that Kenny posted is pretty accurate after all. (I did notice that Mixbus actually exported everything exactly 1 db louder than Studio One. So I had to reduce the Mixbus mix by exactly 1 db in order to get the perfect null)
 
So what this really points too is that any preferences that some may have for Mixbus or another DAW is linked to everything else that the two DAW's may provide. e.g. the dynamics that Mixbus offers built in plus the EQ sounds etc plus the saturation.
 
It concludes that there is no actual improvement to be gained from just using Mixbus alone and no effects and expecting Mixbus to add some summing magic. It isn't.
 
Mixbus can still soften transients but that is obviously coming from the saturation or even dynamics but not the summing engine alone. Which is actually quite good in a way because it shows that with no effects processing used anywhere Mixbus is basically leaving the sound alone.  Unless the null test fails to show what magic Mixbus might be adding but I am not sure on that though. Because if Mixbus was even a tad brighter then it would show in the null test.
 
I am not going to rush out any buy 32C V4 either.  It is not worth it. (Unless you want that EQ of course) The bottom line is if you are good you can get a fabulous (and the same) mix on any DAW. That is something I have always believed anyway.
 
It also makes me think that I can get some pretty cool console emulation going on too with Studio One's built in CTC-1 (if you own it that is) That does sound nice. And you don't have to export or transfer stuff from one DAW to the other. There is actually more control over the console emulation process as well with the CTC-1 compared to Mixbus. e.g. Drive and Character settings.
 
For those trying real hard to route signals from your DAW over to Mixbus and back again I say forget it and just get on with what you are doing with one DAW only! (Unless you really want to use all the other stuff Mixbus has to offer because the summing part of it seems to be neutral) I think there is enough other stuff in all our DAW's that will be able to match most of what Mixbus has to offer.
 
This test that I have just done has changed my opinion of Mixbus. It basically agrees with Soundwise now.

Results did not shock me.  i have been asking for years for someone to show me that mixbus had actually made their music any better. I like mixbus, but I just have never been able to find anything that made porting everything to it worthwhile.  blind tests seem to indicate that a good chunk of the praise is somewhat due to placebo effect.