Linnenberg MAESTRO Owner's Manual page 8

XLR connection use a shared ground for both channels. The only
advantage of the XLR connection is the avoidance of shorts when
plugging in and out a phone jack. Moreover, this is the reason why
one should place the MAESTRO or any other headphones amplifier
with low output impedance in the "MUTE" mode when changing
the headphone connection.
Here is the corresponding pinning:
¼ " phone jack
L GND
R
(Figure 4)
8