just in case anyone else comes up against this, your monitors must have independent volume controls for use with th MR816 otherwise you will get this hissing problem. There is no control after the D/A stage on the MR816 which outputs at full volume hence the hissing sound in the monitors. Fortunately for me, the ADAM P22A has small attenuators on the back that you can dial down using a screwdriver.
Um sorry no.
Press either of the rotary encoders on the MR816x until the assign light is next to master.
then turn it down with the rotary encoder.