Maybe I'm dumb, but wouldn't this be more likely to be an issue with the sound card or interface?
 
Just tested by pulling in mono audio file and panning hard left.  Checked both in speakers and in headphones.  I did not hear amything on the right.

I just tried this with my Babyface and I do indeed hear a little signal coming from one speaker when the track is panned 100% to the other. It's very faint and I can only hear it when the other monitor is turned off but it's definitely there. However - and I don't know what to make of this - if I play music through, say, Windows Media Player and use the RME's Total Mix interface to pan the signal 100% to one side, I DON'T hear any traces of the other channel in the silent speaker. Not even when I crank the volume right up. On my software playback channel, I have the width set at 1.00 and the FX send is totally off. The FX send on my main is also all the way down. 
 
It would be interesting to know if you can reproduce this problem in other DAW's. Right now I only have Sonar installed. 

darkcatalyst
 Xonar X3 build 239

I just noticed this. If this is X3 Producer, build 239 was X3c, update to X3e http://www.cakewalk.com/Support/Knowledge-Base/2007013361/SONAR-X3e-Update If it is not Producer make sure the about screen shows X3e for whatever version it is.
 

scook

darkcatalyst
 Xonar X3 build 239

I just noticed this. If this is X3 Producer, build 239 was X3c, update to X3e http://www.cakewalk.com/Support/Knowledge-Base/2007013361/SONAR-X3e-Update If it is not Producer make sure the about screen shows X3e for whatever version it is.
 

I'm running X3e and it definitely happens there. 

I understand but I am not sure your test actually proved anything other than RME's Total Mix when panned hard to one side plays only on that side. I would guess if SONAR sent a stereo signal to Total Mix the interface would react the same way.

konradh
Maybe I'm dumb, but wouldn't this be more likely to be an issue with the sound card or interface?

Not at all dumb. Haven't measured crosstalk on every interface in the world, but -55 to -60 dB is not uncommon at both higher and lower frequencies. Crosstalk also varies with levels. It can also happen outside the interface, e.g., cables being close to each other.

Have this only with bus input pan, no crosstalk when using output pan

@swamptooth thanks for the details - no luck after adjusting all those settings.  I like the Babyface a lot, it's definitely a step up from the Firestudio Project.
 
After a bit more testing, to summarize my issue: 
 
With the RME, I can hear the bleed during playback.  Bleed is also present in mixes.  With the Firestudio, I can't hear the bleed during playback, but still hear it after mixdown.  I had been experimenting purely with output pan, but input pan doesn't seem to make a difference.
 
I entertained the idea that the crosstalk is introduced by the device I'm playing the exported tracks out of.  I've tried with my PC soundcard, my Nexus 5, and a separate laptop - crosstalk happens across all three.  Tried a variety of headphones and speakers, no change.  But the main reason I think it's not the device later in the chain is that I'm able to hear the bleed during playback with the RME in SONAR.
 
Maybe a dumb question, but the mixdown process is purely algorithmic, correct?  So no hardware should have any bearing on what happens during the export itself (other than the CPU).  Just trying to figure out any further avenue to test/explore this issue.

ok. silly question, but what happens when you pan the master bus output in sonar 100% left?

Pretty legit question considering the oddity - no change from center, still slightly audible in the right speaker.  Perhaps somewhat interestingly, panning the master hard right actually eliminates the sound, though that also makes sense in a way.