Track + Trilian (Stereo Bass)> Bus [ Guitar Rig5 Bass ] = mono
Trying to route synth stereo Bass to a stereo bus, but when it passes through GR5, the signal is mono. Track and Bus are stereo, and GR5 Preferences are set to load stereo, so what could be overlooked? hmmm. (if GR5 is bypassed the signal is stereo)
 
there is a triangle icon (for additional options) on the right side of the bass amp STEREO!
On a real amp ALL the knobs are visible. lol

Personally, I'd treat any bass track as mono.
 
Your mileage may vary 

Are you sure Guitar Rig is set to stereo? Are both the L and R buttons at the top lit up? It defaults to mono. This is what it should look like if you want stereo. 
 

randyman
Personally, I'd treat any bass track as mono.
 
Your mileage may vary 

Actually it's quite nice to give the top end of a bass some stereo spread. Just another color in the palette!