If I have a stereo sample and change the track settings to be mono - L +R Mixed, it is much quieter than the original stereo sample. This also happens when sampling, sampling set to stereo is noticable louder than when it is set to L+R Mixed.
Why is that? On my MPC 1000, I remember the opposite behavior, with L+R Mixed being louder than Stereo. I’m wondering if I have something set up wrong. Changing from stereo to mono requires a complete change in mix levels.
I experience the same. I just turn up the track levels about +2.
I would also have expected the opposite when going from stereo to mono (mixed L+R) - that it would be louder. At least for the most part.
It does require change of mix levels when you switch from stereo to mono, as you point out.
If it acted like you would expect (signal becoming louder) it would still require this though. Just reversed.
Be aware, that if wishing this to be fixed, it would only be fixed for future projects. Existing ones would then require change of mix levels, as they will all be affected.
The optimal solution, would be that levels would remain the same when switching from mono to stereo and vice versa.
I am not sure this is even possible though.
One signal might be louder as a result of constructive interference. Another might be lower because of phase cancelling.
All depending on the individual audio material.
Not sure I even know what I prefer myself.
Just some thoughts I wanted to put out there.
Good topic 
the mix function is as simple as: M = (L + R) / 2 but as stated above it depends on the phase relationship between channels on how that will affect the mono level. e.g. if you had the R channel inverted you would hear a slightly out of phase stereo signal but the mono sum would be silence…
Thank you both for the insight. If mono is just (L+R)/2, I think it makes sense that stereo would sound quieter. I’m typically sampling vinyl or media with multiples sounds already panned across the stereo field. As an example, if I were to sample something that had drums panned hard right and piano panned hard left, when summed to mono the volume of each element would be halved. Thus the piano and drums would sound half as loud out of my speakers, right?
Or is it more complicated than that because half the original volume of the piano would be coming out my L + R speaker vs just my L speaker… I guess some of this also comes down to how we perceive volume as well. I started this by thinking I understand, but may have just confused myself more.