jon,
according to the information above pulled from core-sound's website (and that may be taken with a grain of salt, from what i understand
), hte dynamic range of the adc only equates to approximately 16.5 bits, although it is in fact a "20-bit" adc chip.
i'm not sure if that means that effectively only 16.5 bits are being transmitted and that there's a "natural" dither occurring during the output to the digital device, or if there is actually a truncation occuring removing the least significant bits, which are essentially empty due to the adc only truly representing 16.5 bits of data.
just food for thought...
i've heard glowing praise about your work and am excited to be able to lend even a small hand in your efforts. the ad-20 truly is a great piece of equipment, very small and portable, low cost, and high quality.
your proposed modifications would make this unit that much better. just an fyi, there is not much available room inside, unless perhaps you were to rip out the gain stage guts and only keep the adc, though they do appear to be on the same circuit board...not that i know too much about electronics, but that's an amateur observation.
good luck!