| br>I have a Focusrite Clarett 8 Pre, so not the same interface you do-but had the same problem when I first got it. Here's what I did-with your interface powered on, launch the "Focusrite Control" app on your computer (free download from Focusrite site if you don't have it already). The audio mix feeding the headphone jacks is determined by the settings in Line Outputs 7-8 and 9-10 (may be different # outputs for the 18i20-look for the ones with the headphone icon).
Click on the wide gray box between the blank name box and "STEREO" switch, and instead of a pair of DAW playback channels or hardware input channels (which it is likely set to now), select "Custom Mix", then turn up the software faders and set the pan sliders for the inputs you want to hear in your phones. You can also turn up the lower row fader for DAW playback, so you can listen to that along with your mix of any live inputs when overdubbing.
Oh-and make sure you are using outputs 1&2 to feed your monitor speakers/amp-the software gain fader for that should be green (disabled), meaning it's under hardware control only (monitor knob on the front of the 18i20)-I have a custom mix set up for output 1&2 as well, including all hardware inputs and the DAW playback.
To be on the safe side, or if you need/want to experiment with the software app settings, keep monitor and headphone volumes down. Just turn them up enough so you can tell when you've got what you want. I use a pair of outputs to feed a hardware stereo delay unit, and monitor it on a pair of inputs-just set up a custom mix for those line outs and was careful not to send the delay ouput back to it's own input (keep those faders down all the way or mute them). Typing in names for all your inputs and outputs and saving the snapshot with a name like "Std Setup" once you get everything set the way you want will make it easy to restore later. br> br>