Speaking of distortion, I have noticed with the MOTU interface that there is noticeable distortion when sampling a simple sine wave. This is sampling at 24bits/48kHz.
I've made sure that the distortion isn't present in the original signal. The distortion takes the form of noticeable harmonics of the fundamental frequency, especially the 5th 7th and 9th harmonic.
It's not noticeable with a complex signal, but obvious and audible with a simple sine wave. Never figured out whether it was user error, or just limitations on the hardware itself.