The CD format offers 16 bit which means a dynamic range of 96 dB and distortions which cannot be lower than 0,0016%. A 24 bit signal offers a dynamic range of 144 dB with theoretical minimum distortions at 0,00001%. This is not possible to achieve in real life. The best today AD converters offer dynamic ranges from 120 dB with distortion figures about –110 dB THD. Lots of losses have to be faced during recording, editing, mixing … Digital attenuation is done by shifting the signal from MSB (Most-Significant-Bit) in direction LSB (Least-Significant-Bit). Shifting a complete bit in LSB direction (and replacing it with a 0) means 6 dB attenuation.
When a 16 bit CD signal is input to a 24 bit DA converter, this signal may be attenuated by 6 dB x 8 Bit = 48 dB = factor 200:1 WITHOUT changing anything from the original data. We learned from the above that also a real 24 bit signal carries a maximum of 20 “senseful” bits - in practice there are no more than 18 bits. So, also a 24 bit signal may be attenuated by a minimum of 6 dB x 4 Bit = 24 dB = factor 35:1 WITHOUT doing any harm to the original data. And– for our opinion - digital attenuation is the best what can happen to a signal (except not being attenuated). Of course provisions should be made to adapt different working levels in the audio chain.