I can see noise being constant (assuming that dither noise is negligible, via noise-shaping, etc.). The SINAD floor includes both noise and harmonic distortion, so it is generally a question of THD as well, not just noise. Of course, if THD<<noise then this is irrelevant. This seems to be the case for the D90, according to Amir’s measurements. (-130dB THD compared to -120dB SINAD).
Understood, thanks for spelling it out more clearly. And -30dB is anyways a lot of attenuation...if one has to go lower regularly, then a preamp or less muscular power amp is called for.