I use multiple performances and rigs when live - my trick to get them all around the same level (volume wise) is to do the following:
1. Connect via SPDIF to an audio interface
2. Load up Reaper
3. Use a plugin that looks at the real-time loudness via a loudness meter (I use Ozone)
4. Record a riff with the looper (I use the opening riff from Stiff Competition by Cheap Trick)
5. Normalise the rig volumes by checking the loudness meter for each rig (you'll need to reset the graph to double-check)
Seems to work well for me!