Flac, replay gain and ...... Options

[UncleMcFlac]

Hey , can anyone help me? i use foobar ,and have tons of lossless files, i see the function replay gain but im just learning to use it , heres what ive done to tracks that sound way too low , ive increase the decibels and converted the flac file to flac (click on file > convert to> replay gain > move the preamp together> convert), so when i do that is the process irreversible?
and can someone teach me how to apply replay gain? and explain this thing to me ?
Note: since im a noob , what i do is keep my replay gains between -6db to -9db , there are some track like "one direction - whats makes you beautiful" that are about -12 db(that too darn loud), and the "killer - mr brightside" is that loud too but its not bad because it is rock (rock known for distortion) but the OD song just sound like a hiss fest(the person who produce it should go back to mastering or producing school), also the track i mentioned ive turned them down using the method above but i wanna know if that "technically" degrades the song

[mjb2006]

It may help the OP to realized that "apply ReplayGain" means something different than "add ReplayGain tags".

Normally you want to just add the tags. They tell a ReplayGain-aware player how much to adjust the volume in order to get to a reference level. The player will "apply" the volume change (if so configured) to the audio data on its way to your speakers or headphones. It won't be changing the audio in the original file.

If you "apply" the ReplayGain to the file, though, then this does change the audio data in the file, such that it will play back at the reference level no matter what player you use.