Spotify - BAD sound quality (compared to Youtube Music or mp3s)

[shhhhhhhhhhh]

Hi. I have (I think) an audiophile setup:
Hifiman Sundara headphones (I consider them the best under 600 EUR) and my friend's Sennheiser HD650 headphones, Fiio K5Pro USB DAC + headphone amp.

Both on Android and Linux App and Web player have the same issues:
The same tracks (I used Patsy Cline - Crazy for reference but tested a lot more) sound overly bassy, muddy and lacking definition in highest octaves when playing through Spotify.
Those reference tracks played through Youtube Music sound almost as good as the 320 kbps OGG.
On Spotify (Desktop app, Web player, Android app) it all sounds basically the same: bloated bass and recessed treble.
I have selected the best possible audio quality (and EQ is off) within Spotify but the difference between High and Medium is undetectable.

I am currently new to Spotify so I don't have a Premium plan but unless I'm sure it pays off I'm hesitant to subscribe.
Am I the only one with these issues? Did anybody benchmark sound quality and found out the same music sounds almost like playing through a ghetto blaster?
I'd love to subscribe (great integration between devices, a huge library of tracks) but the sound quality puts me off tremendously.

Thanks indeed for any help or sharing your experience.