Decibels are a relative measure of amplitude or energy. With audio dB is related to loudness.
The reference for digital audio is 0dBFs (zero decibels full scale) and that’s the “digital maximum” and digital dB values are normally negative. Internally, it’s as high as you can “count” with 16-bits or 24-bits, etc. (Everything is automatically scaled when you record or play-back, or when you load-into Audacity, so a 24-bit file is not louder than an 8-bit file.)
If your current peak is -6dB and you amplify by +3dB your new peak is -3dB.
Audacity has pre-scanned your file and Amplify will default to whatever gain (or attenuation) you need for maximized/normalized 0dB peaks. For example, if Amplify defaults to +6dB, your current peak is -6dB.
I don’t know what ReplayGain is using for it’s reference but it’s some kind of “loudness” measure and it’s NOT the peak dB level.
Audacity uses floating-point internally so it can go over 0dB, but regular WAV files, CDs, analog-to-digital-converters (recording) and digital-to-analog converters (playback) are all hard-limited and you’ll get [u]clipping[/u] (distortion) if you try to go over so 0dBFS is considered the “digital maximum”.
Peak levels do not correlate well with perceived loudness. Sometimes RMS dB level (a kind of average) is used to approximate “loudness”, but ReplayGain uses a more advanced algorithm. There is an RMS spec for audiobooks. And in the case of audiobooks, everything is spoken word so matching RMS levels works well to match volumes.
A dB change will affect all of the dB measurements/calculations by the same amount. If you decrease the volume by 6dB, the peaks will go down by 6dB, the RMS (and average) will go down by 6dB and the acoustic loudness will go down by 6dB, etc.
The reference for acoustic loudness of 0dB SPL (sound pressure level) is approximately the quietest sound that humans can hear so SPL levels are normally positive. There is usually no calibration between digital levels and acoustic levels but there is a direct correlation. (If you reduce the digital level by 6dB the acoustic loudness will also go down by 6dB. Of course, that assumes you don’t touch the volume control or get closer to the speakers, etc.).
Perceived loudness is related to the short-term average [u]weighted[/u] to account for the fact that our hearing is most-sensitive at mid-frequencies, and [u]adjusted[/u] for the ear’s varying frequency sensitivity at different volumes.