I could discover M15 for 45 minutes today.
Tried with MAD24 and Plato, and Zeus XIV CR (custom retuned) with PW1950. Ment to be 10 and 8,5 Ohms.
Speed and fluidity is incredible.
Build quality seems very high, screen is superb even though a little dark (even with high luminosity), and FiiO music is very nice to use.
Given it's form factor and weight, handling it is a real pleasure.
If we talk about sound now. Well, I have to admit I'm impressed.
Dynamic is excellent, tonality great, and power "feeling" just at the highest.
Bass are dense and so deep, mids are on the edge between technicality and warmth.
The stage is rather close but it's not short of air. It's pretty wide too.
I have the feeling it's in the spirit of the CAYIN N6ii with the T01 module but even more on the technical side. To be honest I prefer the FiiO.
Really good for price.
On the other hand, it heats up quickly, and the wheel doesn't inspire me too much confidence when inside a pocket.
What about the bass with the balanced output impedance at 3 Ohms?
Well clearly it's exaggerated, but they're pretty clean. It's not an inaudible blister.