Okay, I downloaded the manual and according to specs it would appear those amps have 29 dB gain for both RCA/XLR. If they are different, such as many other power amps, they would specify two numbers such as 29 dB for RCA and 23 dB for XLR, or 1.2 V RCA, 2.4 V XLR.
Marantz is one of those amps that have two different gain (or input sensitivity) such that the balanced to unbalanced input sensitivity of their power amps would be 2:1 (that is, 2.4V/1.2V, just an example), or gain ratio of 23 to 29 dB, and their preamps output would have a reverse ratio such that the balanced output of their preamps would be rated 2X that of the unbalanced output (example: 4V/2V, or 2.4V/1.2 V etc.)
Since you are using the balanced output of the AV7706, the Emotiva is getting 2X the voltage relative to that from the unbalanced output (RCA). That would represent a 6 dB difference.
The best/easiest way to solve this is by simply using the unbalance/RCA outputs of the 7706. Again, unless your interconnects are really long, like 4 meters, there is no need to use XLRs.