Amp features a selectable input gain. This can be set via a push switch on the rear of the unit. Use a screwdriver, pen or similar device to locate and push the gain switch.
Pushing the switch to its depressed position configures Amp's input gain structure to be compatible for use with an Optimal Audio Zone processors line outputs (20dBu max).
Pushing the switch again releases it to its upper/outward position, this configures the input to 32dB gain, for use with traditional professional audio equipment.