I am in the process of altering the volume on my files (Wav, Flac, MP3 and AAC) in my library so my albums and compilations all end up being a similar volume level.
Files that need reduced volume, I simply Amplify by anything between -0.5 and -3.0. These reductions are done on files with quite high volume levels that generally have a fair amount of compression. Are there any playback problems presented with reducing the volume level by as much as -3.0 or more ?
With files that need an increased volume level, I either Amplify or Soft Limit (to avoid clipping) by anything between 0.5 and 5.0. These increases are done on files with relatively low volume levels. Are there any playback problems presented with increasing the volume level by as much as 5.0 or more ?
I am also wondering are there limits to how much I should increase or decrease the amplifications and is one method (increasing or decreasing) better than the other ?
Changing the volume only scales the waveform bigger and smaller, there’s no quality loss associated with it, as long as the stuff you care about remains between clipping and your device’s noise floor. Having the signal sit anywhere between 0 and -30dB should be fine on all devices.
Limiting to prevent clipping does distort the waveform a little, so it’s best used to only shave off some stray peaks.
Note that volume is not a very meaningful measurement when it comes to things sounding equally loud, which is your stated end goal. For that, you need to look at loudness, which can be measured in LUFS or dB RMS, and which the loudness normalization effect is for. If you want to do it by hand anyway, make sure that the inner (light blue) waveform is equal across all your files.