The issue; you want to bring existing audio (reference audio, such as an existing song) into Reason for comparison. The Flower Audio’s Loudness Meter device (shown in the photos) can be used for referencing, but it’s used to switch from the audio in your setup versus your reference audio; you can’t compare the two side-by-side.
The issue with trying to do normal referencing is that the sound source will appear under your mastering suite, getting extra mastering effects placed on top. Simply turning the fader levels of your reference channel down doesn’t do the trick as you’ll get lower levels that are still being colored (altered) by your mastering suite setup. Some people may Continue reading Reason: How To Wire Reference Audio To Bypass Mastering