So, several things going against you here. First, as what I assume is a competitive move, Sonos doesn’t publish electrical diagrams of their speakers. Second, they discourage users from opening them by invalidating any warranty as soon as the device is opened. Third, they don’t even provide an authorized service option. Fourth, the market Sonos appears to be interested in is the “plug and play” market, and not the audiophile market where such statistics are thought to be significant, and worthy of argument.
Since this is a community site, and not a direct connection to the Sonos design team, you’re unlikely to get much solid information here. I would guess there are other places where people talk about disassembly and repair/replacement.
In the Advanced systems area of the forum, there have been occasional posts from people who have opened them, perhaps you might get further information by posting in one of those threads, although to my faint recollection, they’re mostly about power supply replacements, but I haven’t followed them too closely.
A/D chips are commodity items and the math they use (Nyquist/Shannon) has been around for almost 100 years. If you ever find one that is not transparent to the source, it is because the manufacturer is deliberately coloring the sound.
IOW, nothing to worry about.
So I assume then that there’s no point in using a separate DAC if I were to add a Port to my legacy pre-amp/power-amp combo?