The manual explains each button and knob:
The amplifier responds to Denon RC-850, RC-857, and many universal Denon remotes from the 1990s–2000s using RC codes for the PMA series. Denon Pma-655r User Manual
Source Direct bypasses the tone and loudness circuits, which often add a slight gain and frequency contouring. This is normal and indicates a purer signal path. The manual explains each button and knob: The