MSB Technology The Reference DAC User Manual page 7

Additional Analog Inputs and Outputs
In addition to the primary output module, additional analog inputs
and a secondary analog output can be added.
Isolated
Analog
Output
Isolated
Sub Analog
Output
Additional
Analog Input
Provides a second analog output as single-ended or
balanced.
Only one analog output module can be added per
DAC.
Provides an isolated sub-woofer output as single-
ended or balanced. Only one can be added per DAC.
Provides an extra analog input connection as single
ended or balanced. Multiple inputs can be installed.