Rich,
When you tried the 'no effects' approach, did you remove the effect completely from the bus?
Also, if this problem was happening for me, I'd try the following (just to make sure that the issue extends beyond the file I'm presently using) ...
1. Start a new RB worksheet by exiting RB and re-opening it. This will help clear anything that might have become lodged in the computer's memory (hopefully).
2. Open Stylepicker by right-clicking on the style name window.

3. When Stylepicker opens, select any style with a single right-click. After doing so, click on the "Load Song Demo" button.

4. In Realband 2015, once I click on the "Load Song Demo" button, RB automatically exits from Stylepicker, generates the song demo and then begins playing the song.
If this doesn't happen, click on "OK" to exit Stylepicker and then generate the song. Depending how many RTs are in the song, it could take a minute or two to fully generate. Once the generation is complete, play the song.
Do not add any effects to the song.
5. When the demo song has been generated, merge the audio tracks into a stereo wav file. It's important to use the wav format because that is the base format from which mp3 and wma files are created.
6. Test the newly created wav file in Windows Media Player and see if it sounds stereo.
7. In additon, once the wav file has been created, create a new worksheet in RB (using "File | New") and then import/open the newly created wav file into RB. Check to see if it is stereo (or not) in RB as well.
Undertaking the above process will give more insight into the problem and help isolate what might be causing this issue.
Regards,
Noel