FreeFlyBertl
 
 
 the sound is often very different when using single out compared to multi-out.
 
reason #1: summation inside VST maybe slightly different than in Sonar due to not having faders exactly at same level, clippling that may occur in the VST plug without noticing (not all VSTs have clip indicator in the "mini" mixers, and if they do what's their threshold?) 
 
reason #2 (the major reason): all plugs that have an internal "mini" mixer also have a "mini" master bus and VST presets always have (quite optimised) FX on master bus (e.g. NI drum VSTs) thus they compress more, sound punchier, sometimes even add room FX, etc. 
 
so by going to multi-out, you circumnavigate that "mini" VST master bus and commit to creating that final VST sound yourself ... however, you can usually also route that VST master bus to a separate Sonar track, so you can blend that with your own mix (e.g. do some sort of parallel compression by blending that compressed VST master out with your individual tracks) ... remember, there are nor rules - anything that sounds go to you is allowed

So, you're saying that using the single output actually gives you better sound? And that going multi necessitates (and presupposes) excellent mixing skills, because you are bypassing the optimized internal Fxs. I need to use that built-in optimization. For example, EWSO is a wet library. GPO and Vienna are dry libraries. I balance them bu using the native verb on Play, which is, as you have pointed out, optimized. I don't send my Play synths to Spaces, as the verb I use there, Northwest Hall, is actually the same as the EW Hall that is the native verb on Play itself. That way, I can use this Hall for all my synths. I do send the other libraries there. so everything ends up in the same place. (I have to use some of the native verb on GPO and Vienna for some instruments.