Yes I'm well aware of that but that isn't what dynamic range is referring to in the above. The only thing you affect by reducing the volume in a digital signal

Yes I'm well aware of that but that isn't what dynamic range is referring to in the above. The only thing you affect by reducing the volume in a digital signal is increasing the noise floor, or decreasing the signal-to-noise ratio. 16-bit audio already has a noise floor well below what a human can hear for a recording which has a "normal" volume as you might find on any CD. On top of this, when you use replaygain in your audio player, the digital volume adjustment is applied to a 24bit signal, and Windows (or whatever OS) has a 24bit or 32bit float signal path, and then most or all DACs are 24bit (for example newish Realtek integrated audio). So it really makes no difference to effective SNR either.