XLR carries a hot signal on one conductor and an inverted hot signal on another conductor. A diff amp compares the two signals and amplifies the difference. A +5 volt compared to -5 volt is a difference of 10 volts. That's the 6 dB gain.
There should be no gain if Luxman followed the AES48 XLR interface standard, so apparently they do not follow AES48. Read the posts by @atmasphere (Ralph Karsten) in this thread:
https://forum.audiogon.com/discussions/when-to-choice-xlr-over-rca-ics
The consequence of not following AES48 is that some of the benefits of using balanced interconnects are lost. However, XLRs may still provide some benefit and you'll need to compare it with using RCA on your system.