MSB Technology The Reference DAC User Manual page 16

Ground Jumper IN - Basic Operation
The Basic Operation provides isolation only for the DAC. This gets
you half the protection available. Be sure the jumper is in place
between the Chassis Ground and Amplifier Ground. This is the
shipping configuration. NEVER OPERATE WITHOUT THE JUMPER OR
A GROUND WIRE ATTACHED.
Ground Jumper OUT - Enhanced Operation
The Enhanced Operation provides isolation for both the DAC and the
amplifier. This gets you the full isolation available. With the jumper
disconnected, connect the supplied ground wire from the AMPLIFIER
GROUND lug to the chassis of the amplifier. Note this connection is
dependent on the amplifier so you will have to look for the best place
to attach the wire. Generally the easiest place would be to loosen a
screw on the Amplifier Chassis and slip the open Spade lug under the
screw head and tighten the screw. The only other place a true ground
may be found is on the ground pin of the power connector to the
AMP but this will not be easy to connect too.