Stereo Left and Right swapped inside DAW only - huh?

Hello, all; 
I have a problem that will probably have everybody on their backs, hooting. 

Inside Sonar (X1 PE) my Left and Right Channels are swapped the wrong way round. 
When I pan to the right, left loudspeaker gets louder and vice versa, same with pulling only one side of a stereo strip. 
Before you say "swap left and right out plugs on your audio interface, dumbo" hear me out.  

This is how my listening signal is set up: 
PC -> Audio Interface -> Stereo EQ -> Monitor Switchbox -> Loudspeakers

This is what I checked: 

• Jacked headphones into Audio Interface and used Windows Sound test. Left is left and right is right. Left and right LEDs on EQ and MonitorSwitch lighten up accordingly. Louddspeakers play sound correctly as well. 
• Let some stereo stuff play in Sonar, didn't touch anything in Sonar; just checked if I had any cables connected crosswise. Nope, all good, all the way from audio interface to loudspeakers. When I do something (raise or lower volume, switch off left or right etc.) at any point of the hardware, the correct side responds.
  So I assume the physical side of things is all set up as it should be.   

Only inside Sonar controls affect stereo sides the wrong way round. 
I've skimmed Preferences up and down but didn't find anything like "exchange left and right stereo sides" or similar. 
What am I missing / doing wrong? 

Thank you very much for any light you may be able to shed. Best - Saur