My first impression was mixed - pleasant soft sound signature (mellow, full bodied but too dark sound). In comparison with M11 Pro, M15 demonstrated inferior bass control. Detail retrieval and separation was also not as good as with M11 Pro. M15 had narrower sound stage and lower sound stage accuracy.
In other words, my first experience was not promising, considering that M11 Pro sounded outstanding right out the box.
After 30 hours of burn-in at medium volume using different headphones connected balanced, everything changed dramatically.
2nd time I listened to M15 it delivered terrific separation, musical flow, lots of detail and very wide soundscape. Superior to M11 Pro in every aspect.
Tonality also seems right after burn-in. In comparison, M11 Pro sound is dryer, less tonally rich. Now M15 sounds more like my reference system (Denafrips Pontus + Drop AAA-789), when at first it was more like not the best tube amp.
Comparing with M11 Pro - M15 is much more resolving DAC/DAP, it has properties of hi-end stationary equipment in small package - effortless musical flow, micro/macro details, dynamic contrasts, lots of air, stable space, seemingly absolute instrument separation and no noticeable digital glare, like listening to truly high-end DAC.
Some tracks, like Beija Flor by Jim Hall Quartet, deliver dramatic difference between M15 and M11 Pro making M11 Pro not engaging in comparison (make you wish to push stop and return to M15).
Difference between M15 and M11 Pro is not as noticeable with IEMs as with e.g. HD800 or Ananda, even with IEMs like Westone ES60.
M15 is still better, especially in tonality, but it does not justify 2x price, unlike with stationary headphones.
M11 Pro is an outstanding product on its own, I think it created new reference of performance for portable audio at reasonable price. Now M15 sets reference for performance and also value for those who are looking for best possible portable sound.
Even if your budget allows you to consider more exclusive products you need to experience burned-in M15 before making decision.