From manual:
Balanced XLR Input Jacks
In most systems balanced XLR connections will give you the best sound. If your stereo preamplifier has balanced XLR output jacks, we recommend that you connect them to the A 21+ XLR inputs. Refer to the Balanced and Unbalanced Lines in the Technically Speaking section for additional information about why we recommend using balanced lines.
Note: Using balanced XLR input connectors results in a 6 dB higher volume level compared with using the RCA input jacks. This is a noticeable increase.
Unbalanced RCA Input Jacks
Use these inputs if your preamplifier doesn’t have balanced XLR output connections or if you simply prefer to use unbalanced connections.
Balanced/Unbalanced Selector Switch
Place the switch in the position for the input type you will be using.
Note: The Balanced/Unbalanced switch is not an input selector. Its function is to optimize the signal to noise ratio for each type of input. You should not connect both the Balanced and Unbalanced jacks at the same time with the expectation of switching between two different devices such as a preamp and surround processor.
Balanced XLR Pin Configuration
The A 21+ XLR jacks conform to the industry standard of:
Pin 1: Ground Pin 2: Positive (+) Pin 3: Negative (–).
