While I can't speak to either the iPhone or the Walkman, the quality of the digital-to-analog converter is significantly more important than raw horsepower or other capabilities. Where they're analog devices, there's much less to be said about the "it's just zeroes and ones" argument.

Some older devices I've owned had good-quality DACs - a Palm Treo being the best. My current ThinkPad has a downright awful DAC, to the point where music sounds _better_ over my Bluetooth headphones (Audio-Technica ATH-M50xBT) than when plugged straight in. The quality issues are even more egregious for the cheap dock I use for my work MacBook Pro, which puts out so much electrical noise over the headphone jack as to be annoying even in in Zoom calls.