Behind the MOD, beside each input, there's a red button to select between Line and Instrument and a black button to adjust gain. Beside each output, there's also a gain control. See the picture:
At each MOD display, there's a "X" icon that will turn on in case the corresponding input or output has it's volume level clipping. The 1st display represents the 1st input. The 2nd display represents the 2nd input. The 3rd display represents the 1st output and the 4th display the 2nd output.
See below some tips on how to use these controls to get obtain best sound quality with MOD.
1. Selecting line or instrument
After connecting your instrument or audio source, select the option line / instrument properly.
The Line option - red button pressed - is used for active instruments (powered) - bass and guitar with active pickups, keyboard and MP3 player.
The Instrument option - red button not pressed - is used for passive instruments (not powered), like bass and guitar without active pickup and dynamic microphones.
This selection garantees that the input impedance will match the instrument / device.
2. Adjusting the input and plugins' gain
Keep an eye on the clip warning icons.
A good starting point is to leave the input gain at minimum level, leaving enough headroom for the plugins to work without clipping. In case a connected instrument produces a very low signal, like the dynamic microphones, you should raise the gain, paying attention to the clip warning icons on displays 1 and 2, according to which input you're adjusting.
Other then the preamp inputs, several plugins, like tube amp simulators, also have a gain adjustment an so they do a digital signal amplification.
The clip warning icons at displays 3 and 4 will light up when these plugins have a very high gain, and so the signal is already clipped when it reaches the output.
3. Adjusting the output gain
The output gains are available so that the output signal can be adjusted to match the input of the device to which the MOD is being connected: an audio mixer, a recorder or amp. Some devices, like guitar combos, have a high input sensibility and require the MOD output signal to be attenuated, so that the amplifier input is not saturated.
At first, these gains can be at maximum level and, in case the signal is too strong for the connected device, the output gain should be lowered.
The output gain can also be used as a master device volume, letting you lower the MOD sound when you don't have access to the amplifier (a common situation in shows).