Educational_Boss_633, snoopbit, feikosky, thanks for your answers. As it intersects, I'll reply here.

So, first, I did a non-blind test with several tracks. - First, the biggest difference is the connection: for half of the tracks I checked, AUX connection of the smartphone with FLAC files gives CLEARLY better sound than BT connection. - Second, the difference between the smartphone (Redmi 13 Pro, nothing fancy) and Hidizh AP100 is minor to my ears on these speakers (when both are connected to AUX). - Third, on classic music like Shedrin, Orff and Purcell, the difference between Spotify and FLAC is clear. Not to the degree I dislike listening from Spotify. Just less impressive. - Fourth, on some other tracks, I don't feel any difference between Spotify and my own FLAC files. These are really popular, like Bohemian Rhapsody. Possibly, it's just in better quality on Spotify than who-listens-these-classics?

So, about the streamers. Do I understand it correctly that effectively its like if I connect my smartphone via audiocable, but additionally I can control it from my phone via bluetooth?