Amp Volume and Rig Volume are just volume (*gasp*) parameters that do not change anything else.
Lesser volume can/will however be perceived differently,so you might need to adjust/compensate with Master Volume.hth
Ouch, the actual problem was, that until now I didn't now there was both an Amp and a Rig volume - I thought they were the same. Found what I needed and the perception at different volumes is clear to me. Problem solved, thanks to you, Don! Much appreciated.