But here's the thing: so far, just using free Spotify 44.1 redbook quality,
Where did you get that info? It's totally incorrect. Spotify doesn't offer any redbook ouput. They have nothing but lossy streaming at a max of 320kbs. And the free service tops out at 160kbs when streaming.
Tidal, Qobuz, Amazon, Apple and others offer 16/44.1 which is REDBOOK CD quality in lossless formats.
Yes, better streamers and better DACs will sound better but you really need a better streaming service to make that a reality.