the question isnt really about the quality of the source or
connection (as established elsewhere) within reasonable limits. The
whole point of using an external D-A conversion (and/or codec) is to
allow one to spend money where it really counts - on the best quality
converter available, as opposed to using the 20 pence codec chip that
comes built into the source.

odd that, you'll find the same 20 pence codec in a far east DVD player as
you likely will in an outboard processor.

Differences will be mostly in the analogue stages, PSU, amplifier, filter
etc. Likelihood of finding a cheap philips or crystal part as the DAC is
quite high. YMMV (I've never taken one apart) but it's not the DAC that
costs money, it's things like transformers, caps and low noise transistors.