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

[dhromed]

There's not much change between the two as far as I can tell. Maybe a dB or two. My experience often disagrees with RG by as much as 10.

Looking up the numbers right now, the Gnarls Barkley album The Odd Couple clocks in with corrections of -7 to a whopping -11 according to RG, but I've adjusted it to -3 across the board.

The relevant Motorpsycho album Trust Us has values of -8 to -10, and one +8 (!). I've adjusted none of it.
Another Motorpsycho album, Let Them Eat Cake, I've always perceived as rather loud, but here RG gives me numbers of -3, -5, -7 and the occasional -8. I've adjusted that to a flat -3 as well.

You can understand why my faith in replaygain is pretty much non-existant. I do wonder, however, if my perception of loudness could be accurately predicted by an algorithm. Such a beast might have to combine the equal loudness curve with the dynamic range and try to measure attacks, if that's even possible.

Alternatively, I may just happen to be hypersensitive to the vocal frequency range. Sometimes a loud recorded singing voice cuts into my ears as ruthlessly as a pure sine wave. Is that easily testable?