I have never written a review for Head-fi, my channel is on YouTube, so I will be very objective.
The M20 reminds me a lot of the Zishan Z5, but with a more intuitive and easy handling. The screen is also better and it has a much more elaborate construction.
The sound in filter 8 is completely neutral for me. The other filters cause small changes, but nothing extraordinary. The sound is excellent and maintains the technicalities of the IEMs.
I used the BQ10 on it, which is an IEM that I love, and the headphones sounded extremely correct, just like I listen to them on my desktop stack. I think this product is worth what it costs, it could be around 95usd to be an even higher recommendation, but I do not regret what I paid, I think it is worth the price.
I didn't experience any distortion at higher volumes and it drove my HE400SE very well at high gain at 4.4.
I still haven't been able to get it to work as a USB DAC. I'm trying to get in touch with the brand to understand what I'm doing wrong or if it's a problem with the product itself.