Hi Sam.
For a stereo feed to and from your outboard device, your audio interface will need to have at least two inputs and four outputs. For mono, you’ll need 1 input and 2 outputs on your interface. The following example is for stereo.
In your DAW, route the signal you want to process to the 2 outputs you will assign to the device. The other 2 outputs will be for your monitors.
The outputs from the device will be sent to the two inputs on your interface. Create 2 audio inputs in your DAW and assign them to the inputs coming from the device. You should now have a flow from your DAW to the device and from the device back into your DAW.
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