Stereo usually has different signals being output to the L and R channels. This assumes the listener will be seated in the centre at all times for the best soundstage. If you listen to some stereo recordings the channels one side may appear to be dominant which isn’t practical if not seated dead centre.
Making the L & R output of the system in Mono allows 100% of the signal to be output to each speaker. It’s the same effect of listening to music with headphones and the soundstage is centred in your head.
For further reading the search could include “ 2 channel Mono”. Many audio manufacturers are selling Mono Block Amplifiers in pairs for this Mono sound stage.
I would be interested in the answer to how to get mono via Audirvana software, my reason for wanting it is that I have damage to the hearing in one of my ears therefore a stereo soundstage leaves me missing sounds in the bad side of my hearing and my preference is to listen through headphones for better detail, therefore I would get a fuller listening experience from mono. I hear the suggestion about putting loudspeakers close together, not sure how seriously that was intended.
Insert an audio plugin to Audirvana which is able to sum to mono. I recommend DMGAudio TrackControl. This is a professional tool, and it’s free! Inside you find a preset called “make mono” - and you are done.