I just check this album (Dark side of the moon). All values are about -6db. If the ablum is recorded properly, thats the way it should be. Specialy in classical there is very often a high (and wanted) differnce between tracks.
Let me write my understanding about replay gain:
Since there is no standart output volume (as it is in cinema systems) replay gain uses a volume refernce (rms-power) of -20db, in humen beeing is 1% output power) as reference. On a calibrated system, this gives you 20db overhead between the average volume and peak output. These values refer to power, so talking in drive voltages, it would be -10db. This is the value replay gain uses as refernce rms-volume.
Listen to an album (eg. Dark side of the moon) this means: using replay gain, the entire album is played back at a volume equal (playback volume - 6db), if rpg is enabled. Looking at the calculated values, they all were in the range of -5.4 to -6.3 db. So, usíng a calibrated system and reducing the output volume by -6db would have the same effect.
Listen to recordings ripped from differned albums behave totally different. Some companies like to make a powerfull sond by compressing the sound and normalizing it to 0db. This gives you a very high rms-power in comparison to other recordings. Replay gain will now compensate for this almost perfect.
As a conclusion to this, album again should only set the entire album, not an single track, since what you hear then, is not what the artist wanted. Specially Pink Floyd were using loud an silent parts as speical effects in their albums. If you like all tracks play on same volume, the use of a dynamic compressor would be better.
But, since the entire album is adjusted by the same value, listing to an album, turning rpg off and readjust the amplifier should give an equal result. That's why I stated earlier, I see no real need for album gain.
Udo