Using EncSpot Pro I've noticed that there is a tag (activating the Lame Tag group) called "Quality Index".

1) How much is "Quality Index" reliable?

2) I've noticed that encoding into an mp3 file using --alt-preset standard mode I get a Quality Index = 78 ; if I encode using --r3mix standard instead, I get a smaller filesize, a lower bitrate, but Quality Index = 88!! : this means that --r3mix is better than --alt-preset standard!?

3) Is "Quality Index" guessed by EncSpot Pro or this information is really stored in the mp3 file during the encoding process?

It's a combination of the -V and -q settings. Pretty much meaningless as a reliable quality indicator. It's stored during encode.

This is also explained here: http://www.r3mix.net/extra/tag.html

Thank you.. R3MIX still rulez!