EQ differences Sonar vs other tools

I don't understand this at all. 
 
My project uses a Sontius EQ effect on the master bus which introduces a standard V pattern. raising low and high end and lowering midrange. For unrelated purposes, I need to export the audio of this project into Adobe Audition. This last time, I wasn't liking the mix, so I added an EQ filter to the exported .wav in Audition, which was also a standard V pattern. It suddenly sounded much more to my liking.
 
My assumption here is that these two EQ filters are additive. For example, if I raise 500Hz by 3dB in one and 2dB in the other, it would be the same as if I raised it 5dB in just one of them and didn't use the other at all. With this assumption in mind, I decided to "add" the settings of the Audition EQ into the Sonar effect, so I wouldn't need to be using both, and also so I would have this sound I liked in Sonar while working on the project. 
 
So, that's what I did. HOWEVER, it didn't work. (I'm using just the one frequency example here for simplicity, but I did this on the whole spectrum.) Adding 3dB to a frequency in Sonar and 2dB in Audition does NOT sound the same as just adding 5dB to it in Sonar alone. Why is that?
 
Semi-facetious postscript: Also, could someone else just do this for me? I sort of don't like messing around with the mix, and I never feel like I'm doing the projects justice. I wish I could just hand the whole enchilada off to someone. For free.

(Side thought: you know what would be cool.... A browser plug-in one could install that can automatically play audio when you hover over a hyperlink. People on this forum could then say "I mean something like this" and include an audio snippet attached to a hyperlink, and the reader, who has the plug-in installed, would just have to hover their mouse over it and that would start playing. Maybe there's already something like that.)
 
I don't know, but all I know is that I like it better after I've added that V to it. But my question applies regardless of what EQing I'm interested in. Shouldn't the two effects be additive? Why can't I sum them up into a single effect?

You are correct to assume that two EQs in series are additive. A boost or dip of X dB on one should yield the same results on another - assuming all variables are exactly the same, and the plugin isn't intentionally adding harmonic distortion (AKA "tube warmth", "saturation", "color", "character").
 
The big gotcha here is the challenge of making sure all variables are exactly the same. This will be especially difficult in your case, because the two equalizers in question do not handle bandwidth in the same manner. The Sonitus is constant-Q and Audition's is proportional-Q.
 
Many prefer proportional-Q filters because they believe they're easier to dial in. But they do make it nearly impossible to set up one EQ exactly the same as another.
 
 

different EQ slopes are probably first on my list of reasons, after the distortion and what not that many soft plugs can add, as Bit said.

Something to watch about boosting bass and treble and cutting lots of mid is that while making the eq change our brains tend to think the more mid you cut the better things sound. But going back later and listening with fresh ears the end result usually doesn't have the same impact and is very tiring to listen to.

I don’t know why this happens, but it’s common enough for Mesa Boogie to warn their Mark Series amp users against making excessive mid cuts in the graphic eq.

tlw
Something to watch about boosting bass and treble and cutting lots of mid is that while making the eq change our brains tend to think the more mid you cut the better things sound. But going back later and listening with fresh ears the end result usually doesn't have the same impact and is very tiring to listen to.

I don’t know why this happens, but it’s common enough for Mesa Boogie to warn their Mark Series amp users against making excessive mid cuts in the graphic eq.

And it's especially problematic with program material, because even a 1 dB cut means you're adding a 1 dB cut to every single track.
 
I think the "smile" curve is really designed for parties. That way the music seems really loud, but there's enough midrange removed so people can have conversations.
 
I often use a much tamer variation, which adds a shallow cut (between 0.5 and 1 dB) around 300 Hz. This can "tighten up" a mix. I guess it would be the "smirk" curve instead of the "smile" curve.
 

Anderton
I think the "smile" curve is really designed for parties. That way the music seems really loud, but there's enough midrange removed so people can have conversations.
 

To be honest I doubt anyone knows the origin of the practice. It’s just turned into one of those things “everyone (thinks they) know” you do with a graphic eq because......reasons. Maybe the one-time fashion for monitoring through NS-10s had something to do with it.

Anderton
I often use a much tamer variation, which adds a shallow cut (between 0.5 and 1 dB) around 300 Hz. This can "tighten up" a mix. I guess it would be the "smirk" curve instead of the "smile" curve. 

Yep, a gentle mid cut can certainly help clean out the lower mids where mud tends to build up in complex mixes. It’s sometimes surprising how much difference even a tiny eq change can make.