Some more information for reading replay gain values. Stage Traxx tries to find the ID3 tag replaygain_track_gain. It contains a volume adjustment in dB to normalize track volumes. So a possible value would be -6.14dB.
You can use dedicated tools like MP3Gain or audio players like for example foobar2000 to detect track volume and write it into ID3 tags.
The volume scale of ST3 goes from -infinity to +6dB. So the highest boost you can do to track is add 6dB. As most songs are mixed down pretty loud, the replay gain values for tracks are usually in the are -10dB to -2dB. ST3 assumes that songs with positive replay gain values are very rare. So we set the 0dB adjustment in Stage Traxx to the +6dB max gain that we can use.
Sounds complicated but what it does is actually nomalize the volume to the highest possible setting ST3 can use. In the end ST3 adds 6dB to the replay gain value and uses that as the initial track volume setting. So a track with a replay gain value of -8.2dB would have a initial volume of -2.2dB. A track with a replay gain value of -1.4dB would have a volume of 4.6dB in ST3.
The advantage is that the volume is as high as possible. The disadvantage is that low volume tracks with positive replay gain values will be capped at a volume of 6dB.