I just joined the M3X crowd... Just bought it from Linsoul. They're having an end-of-year sale plus limited additional $8.00 discount coupon and free shipping.
Anyway, I've been browsing this thread and one question that always comes up is about system wide EQ that works with apps like Spotify and the most common answer is that a few apps have built-in EQ or you have to somehow root your device for system wide EQ.
Well, guys there is a free non-root solution. Install SoundID app from Google Play Store,
more info here. It will run on Android 6 and up,
more info here.
Check to see if your headphone or earphones are supported for custom EQ tuning. SoundID also has a builtin PEQ that can be used in the case their semi-automated tuning doesn't give good results for you. Or you can have their custom EQ and you can setup other profiles (like Harman target) with the PEQ.
SoundID will probably be one of the first apps I install on my M3X when I receive it from Linsoul.
Since I don't have my M3X yet I can't try it. (I did install it on my Samsung tablet and tuned Grell TWS/1 earphones.) Anyone want to give it a try?