In the driver for AMP-D225H, there is only the possibility to send volume as a value, but it will react to increment and decrement commands as well.
This can be helpful if you are using another control interface than Biamp.
The commands are as follows, represented in both ASCII and HEX.
HEX: 4E 45 55 4E 49 54 3D 31 2C 56 4F 4C 3D 49 4E 43 0D
HEX: 4E 45 55 4E 49 54 3D 31 2C 56 4F 4C 3D 44 45 43 0D
<CR> = Carriage Return