nothing is being 'mixed' thru my RME.
 
all it is doing, is playing back what coming out of the DAW.
 
the routing is so simple in the RME, you cannot screw it up.
 
that said,
this is the same setup i've always used...
 
"entire mix"
 
 
 
now, go back and do one, but pick an different source category....
and pick "Mains" (that's what mine says, yours says 'hardware output')
 
 
what is coming out of mains, or hardware output, should be the same thing as 'what you hear'.
 
 
same thing if you were to select 'buses', instead of 'entire mix'....
and further dial it down to a specific bus, the 'master' bus..............

 
 
Ok I did that and now have 3 mix downs using those different options, Sorry, they do all sound identical. 
Sure this is a real basic project and only VST's. I'll try this on a real project with audio, Plug ins, automation etc. 
One factor that is glaringly different is that when you use a proper audio interface in ASIO there will be those outputs showing in the console view. With on board audio there is no "mains outputs"  after the master buss. 

yea, i've had a weird thing a few times,
where i bounce my two track down to a new track, 
solo it,
set the output to mains,
and still hear a bus, usually a reverb send, still playing as if it's routed in....
 
i'll mute the verb bus, and it will go away.
 
i'll go thru the project,
to find the errant routing,
and nothing is wrong.
 
wtf???????????!
 
LOL,
i'm gonna try the null thing tonight

This is my little friend to trouble shoot weirdness. The other day I was editing a midi track and even after I'd move a note I would still hear the clunker? There somehow was a second midi track "hidden"  routed to the same VST. I have no clue how it got there and why it was hidden, Sonar is kind of like that. 
 

I don't know if this is the answer but if you bounce using the mains you get a stereo wave file with main output processing, if you use entire mix it is bounced before the mains I am assuming. So when you use the main outputs to get your mix you are then running it back through the main outs again when you listen to it. Just a theory.
 

I'm not a power user and I could easily be wrong, but...
 
Try using Main Outputs instead of Entire Mix as your source category.
Disable Fast Bounce and enable Audible Bounce and see what you hear.
 
I have never used Entire Mix while exporting a stereo mix.

Cactus Music
This is my little friend to trouble shoot weirdness. The other day I was editing a midi track and even after I'd move a note I would still hear the clunker? There somehow was a second midi track "hidden"  routed to the same VST. I have no clue how it got there and why it was hidden, Sonar is kind of like that. 
 

DO I HAVE TRACK MANAGER IN 6PE?!
 
LOL
 
i've never used it, if i do have it...
i'll go look......

jonnewyork
I'm not a power user and I could easily be wrong, but...
 
Try using Main Outputs instead of Entire Mix as your source category.
Disable Fast Bounce and enable Audible Bounce and see what you hear.
 
I have never used Entire Mix while exporting a stereo mix.

that's what i already did,
selected main outputs instead of entire mix.
 
got something..... weird.
 
there is no 'audible bounce' in sonar 6.
but you CAN disable fast bounce, which i've always done for the last 10 years.
 
all the mixes i've ever done (4 albums and numerous collabs) were done doing Entire Mix

teego
I don't know if this is the answer but if you bounce using the mains you get a stereo wave file with main output processing, if you use entire mix it is bounced before the mains I am assuming. So when you use the main outputs to get your mix you are then running it back through the main outs again when you listen to it. Just a theory.
 

so,
once the bounce is done (the idea is to get all tracks routed to sub busses, all sub busses routed to master, all fx printed to a stereo pair, irregardless of the routing)....
once the bounce is done, it should reside as a new stereo track, which you can solo, and route to your interface, not the master, to hear what you got.......!
there is no loop.

Well I tried the experiment last night an a big project with all the bells and wistles and the results were the same no matter which source I chose as the output for the bounce. I could not hear any differance in the resulting  mixdowns. I forgot to try a null test, I've read about that but not something I've ever had to try. As I said, my mixes are always what I wanted and I do a few things to check this.
 
Often when something is not right it was a track that was routed directly to my interface and not the master.. So therefore the acid test of muting the master. But then muting the master might not catch that 2 bars of cowbell  in measure 56 unless you listen to the whole song!