If you want to run Guitar Rig from inside of BIAB, this should be down to the ASIO capabilities of your interface and BIAB, and how low of a buffer size you can go with to get latencies low enough for your tastes.

When you were using Guitar Rig, did you have ASIO selected as the driver type?

Standalone mode is something altogether different - this would have not much to do with BIAB whatsoever. This would be a capability of your soundcard and how it handles audio from multiple sources inside the PC.

If you use your Mesa Boogie preamp and external effects, this should be easy for your soundcard to handle and will involve zero latency issues.

-Scott