Is really Stereo Lossy and Joint Stereo Losseless?
why Force Stereo and not just Stereo whats the difference?
The better, non-destructive way to apply ReplayGain is rather in the "Other" section of the converter menu, the checkbox called "Scan files for ReplayGain after conversion".
Stereo and joint-stereo are mathematically equivalent. ...
The OP also asked about ReplayGain and the standard DSPs in the Processing section of the Converter dialog, and we got derailed by the joint stereo query.
the quality of the corresponding encoding processes can be different.
halb27, joint-stereo MS (mid-side) IS equivalent to stereo.
The most challenging test sample (if you want to test the difference) is a near-mono signal, fed through a vocal-cut filter after the mp3 decoder. mp3 encoding doesn't assume the use of a vocal cut filter (or any filter) afterwards. If you never intend to do this in real life, it's not a relevant test.