1704 is the only semi available 24bit R2R dac (integrated circuit version anyway), only noise free dac? did he really say that or are you paraphrasing? if he did… bollocks.... another one of his convenient over simplifications/tunnel vision, like the Koss thing.
He clearly only likes R2R dacs and it dominates his decisions, motivating him to choose a 16bit dac to use with digital volume… when there are rafts of 24bit dacs.
ipods use 24bit dacs, they just dont play 24bit files. I think you would have trouble finding any modern device that doesnt, particularly when they do digital volume
the 1704 requires quite a bit of babying to sound its best, just as most high end dacs do, so yes its all about the implementation and frankly it takes a lot of effort for a portable, proper external digital filters, properly low noise analogue references (not just filtered zener diodes) well executed IV stage, clocking and power supply. its also a mono chip.
anyway guys, i'll leave you to it, I cant say i'm not a bit baffled, but hey enjoy!