Hi Jim,
I have Garritan but have not yet used it with RB. I tried it out and all worked fine. What I did was ...
1. Loaded in the demo for a MIDI style (I used J_SLOBAL)
2. I then right-clicked on the Bass track and set Garritan as the track's VST/DXi.

3. I right-clicked on the MIDI track again, entered the configuration panel for Garritan and set it to play a bass on Channel 2.


4. I confirmed it was definitely playing by changing the sounds in Garritan.
5. Next I left-clicked on the bass MIDI track and used CTRL+A to select the entire track.
6. I then right-clicked and selected to convert MIDI to an Audio Track.

7. The wav was generated on Track 10. It needed normalizing, though. You see this by how small the wave-form (on the right of the image) is on the track.

8. To Normalize the wav, I clicked on it, used CTRL+A to select it. I then scrolled down the pop-up menu (#1), selected "Audio Effects" (#2) and then selected "Gain Change".

9. From the Gain Change menu, I used the drop-down arrow and selected "Normalize" (#1) and "Process" (#2).

10. The wav was modified to maximize its output.

Hope this helps,
Noel