oh, one more suggestion, open you RME software and make sure it's not panned in there.

It would be difficult to be certain unless you performed the test I described, or something similar.  I never noticed until I was creating backing tracks for live situations, panning the click hard left and the synth hard right.  In the mastering stage for those, looking at the stereo tracks during the count-in, you can't even see any waveforms in the right channel fully zoomed, but that doesn't seem to matter.  I only noticed originally because I could hear the count-ins with my PA (right channel only) cranked, something I obviously don't want the audience to hear.
 
I guess what I'm saying is you probably would never notice unless you were doing very particular things.

Just did a full reset on the RME settings in case I was missing something, still no dice.

Just dug out my Firestudio Project and the issue is no longer present.  So it does appear to be something with the RME, despite a full software reset.  Sorry SONAR, you're cool on this one.  Thanks for the responses.

at least you know where to look

I have a Babyface so I'll check this tonight. Does it happen in headphones as well or just the monitors?

I'd make sure in totalmix that the reverb return levels and bith the reverb and delay are all the way off. If you're only hearing faint sound that would be my guess.

Also worth checking with a standalone synth to confirm 100% that Sonar isn't involved.

Might have spoken too soon.  During playback with the Firestudio Project I couldn't hear any bleed, but the issue persists after mixing down the audio.  It's almost like the RME is giving a more accurate representation of what's in each channel than the Firestudio Project.
 
@sharke it does happen with the headphones as well.  Would be awesome if you could do that test though.
 
@swamptooth I don't even see reverb or delay options in TotalMix, but I could easily be missing them, it's a pretty busy interface.

take a look at the pic below...  the middle section, software playback is where i'd look first.  click the little wrench on the output channels then the two knobs to the right are width and fx send.  set with all the way up and send all the way down.  the bottom section is hardware outs. same thing here set fx return negative infinity.  then the control room panel bottom center-right you can click the mute fx button. 
or, you could bring up the output fx window and turn off reverb and delay. 
the interface has a ton of options!  been checking out the rme stuff for a future upgrade.  
how do you like the babyface?