My opinion is that if there is a substantial drop in volume when switching from 2 chnl to 5.1 chnl, then it is possible the receiver is using a single amplifier and splitting power off to each chnl. Most of the manufacturers of higher quality receivers typically provide separate (discreet) amplification for each channel.
Most low-end receivers only have a single power supply for all channels of amplification. This is another cause for the drop in volume when going into surround mode. Better receivers have a power supply for the main channels and a seperate power supply(s) for the surround channels.