The Neural DSP Quad Cortex offers some pretty impressive input and output options. With it, you can record the DI signal (the guitar input without any processing) and the processed amp tracks inside a DAW like Reaper.
It is worth noting that I am on Windows, so for Mac, this might be a little different. For Windows, you also need the official Quad Cortex Asio drivers from here. For macOS, no driver is required.
Now, inside your Reaper device preferences, make sure you have ASIO chosen as your audio system. Then, for the driver, you want the NeuralDSP USB Audio Device (pictured). Where I first went wrong was being confused by the input range.
By default, your input range will be Input 1 and Input 2. These coincide with the guitar/instrument inputs on your NDSP. If you attempt to record from these, you’ll notice you can only hear the raw DI signal, not the modelled one.
The first input can remain as Input 1, but for the last (because it works on a range), we want to choose USB Output 4. Our left and right modelled output will be USB Output 3 and USB Output 4. We keep Input 1 because it’s good practice to have both the dry signal and wet (modelled signal). You can reamp your DI tracks, so it’s great to have them around.
Insert a new track into reaper and choose “Mono > Input 1” this is your DI signal we can reamp (if needed). Insert another new track and select the USB inputs which are our NDSP USB outputs.
You should now have two tracks in Reaper. The first track is the DI and the second is the stereo track with your amps and effects.
Going even further, if you do create a DI track, you can specify inside of your Quad Cortex to accept the output from that track back into the device, allowing you to reamp that dry guitar input with a new preset or model without needing to record the guitar again.