Here is a comparison of the spectrum of a 1 kHz sine wave at -20 dBFS, processed with 16 bit noise shaped dither, as displayed in the top image spectrum. The top image represents what happens if you play the noise shaped 1644 source through 24 bit TPDF dither as would be the case with a DSP volume control. As you can see, the 24 bit dither raises the noise floor a bit at low frequencies as the 1644 noise shaped dither is quieter than 24 bit dither at low frequencies. But other than a slight measured increase in noise, there seems to be no measured or audible effect on the material, as the added noise is well below that of any DAC.
