The terminals on the back of the BAM are not positive or negative, they are either left or right channel. Just make sure the left output of the AVR is connected to the left input of the BAM.
In most amps, the right channel can be identified with a red marking, perhaps this is causing the confusing with the +'ve -'ve.
In most amps, the right channel can be identified with a red marking, perhaps this is causing the confusing with the +'ve -'ve.