The idea of “mixing in mono” can help you make the right mixing decisions and there are numerous audio plugins that let you switch from a stereo signal to a plain mono signal, thereby giving you the opportunity to check your mix in mono.
Now let’s have a quick run down of some of those utility audio plugins which allow you to monitor your mixing or mastering session in mono.
brainworx bx_control has a “mono solo button” which allows you to monitor your audio in mono.
You can switch to monitoring in mono by using brainworx bx_solo and the way you do that is you simply click on the button labelled “M Solo”
You can force a stereo input signal to a mono signal using Waves S1 Stereo Imager by moving the Width slider all the way down to zero. Hence the Waves S1 Stereo Imager is a good plugin when you need to check your mix in mono.