When the system was setup as 5.1 was it calibrated - either via an auto setup routine or with the receiver's internal test tones and an SPL meter? It should have been and in that case the SBL and SBR channels were left at their default level - probably zero.
Now you engage Zone 2 and the receiver uses those two back channels. If the channel trims for SBL and SBR are at zero and the other channels were set much lower during calibration that would account for much of the difference.
The system was callibrated with the receivers internal test tone, as best as possible with the ear. BUT, at that time the SB channels were selected as None. You are correct in presuming that the speaker level for the SBL & SBR were left at 0 dB, but the settings for the other speakers were at a much higher level (for the mains it was +6 dB). By the way I checked this today after reading your reply, by changing the SBL & SBR settings from None.
With reference to mattburk's reply-I have checked the manual and the menu, there is no other volume level setting for the other room speakers, other than the setting in common with all other channels.
FYI, after changing the setting from none, the SBL & SBR level settings were changed from 0 dB to +6 dB and then the settings set back to none which has to be done to use the multi-room function. There was no difference.
Keep up the brain storming friends.
Regards