This applies to the Scarlett 6i6, 18i8 and 18i20 2nd gen devices and all of the Clarett range.
If you have a device that uses Mix Control then please see this article.
To effect incoming audio using an external effects processor (i.e. a reverb or compressor unit) please do the following:
Connect your hardware line output into a jack line input on the Clarett or Scarlett, in the following example we will be using line input 3. Ensure your instrument or microphone (sound source) is correctly connected to one of the analogue inputs, for this example, input 1.
Open the Focusrite Control application, select ‘Line Outputs 3-4’ and then deselect ‘Stereo’ on this tab so that the tab now displays ‘Line Output 3’ (see below). Then click the source selector below and select ‘Analogue 1’. Then connect line output 3 to the line input on your external hardware.
In the ‘Device Settings’ tab at the top of your window, ensure that Line Output 3 is not being controlled under ‘Monitor Controls’. If you do not see these options then you will not need to complete this step.
Once configured, you can set the gain for input audio first, then set the gain for your external hardware, and finally set the gain for the input from the external hardware.
You will now be able to process your Audio and record it into your DAW as a 'wet' signal by selecting input 3 or use the dry signal by selecting input 1 (or relevant inputs depending on which you are using).
To find out how to use an external effects processor in Focusrite Control to process already recorded audio, see the article here.