[Top Jimmy]

When referring to recording headroom, it's always the noise floor of the recording medium that is being referred to. For digital, that's the quantization noise. With 16-bit, the quantization noise is usually higher than any analog self-noise. With 24-bit or greater the analog self-noise is higher than the quantization noise.

Aliasing has nothing to do with any noise floor or signal to noise ratio. Aliasing is where audio from above the Nyquist frequency is folded below it without adequate filtering.

With 24-bit, you shouldn't want to maximize the signal to noise ratio of each track. You'll find that the audio doesn't sound it's best when consistently pushed to the top of the A/D converter range. It also makes mixing more tedious as you have to deal with relative levels that can be all over the map.