So which is better in sound quality?
Ok is very difficult.
My dx300 now has only 90 hours of burnin.
Were amazing all changes. First mid bass forward, upper mids recessed and poor dynamic range in treble.
After 70 hours, bass more linear, a lot of air in all spectrum, mids now balanced (about upper mids), and amazing dynamic range.
In comparisson: m15 has almost same soft timbre but a lit bit more body (not necessary second harmonics just more body or weight) in m15, bass in m15 is more balanced and dx300 a little touch of gain in mid bass.
In mids dx300 has more air and better layering but m15 sounds more musical i think due the extra touch of body.
In the begining, treble in m15 was more forward and better detailed, and with more dynamic range. But now, are very similar in low-treble and mid-treble but in m15 we have more body. Nevertheless upper treble (extension) is better in m15. Percusions sounds better in m15.
In general sounds similar but m15 has more weight/body, more balanced bass and better extension in treble. On the other hand dx300 more air, more space sensation (soundstage).
About power... let more days. But now, for me m15 has better in sensation of dynamisn in mids and treble (bass not).
Overtone saturation is very subjetive, many people says that dx300 sounds natural, but i dont think that. My astell and kern sp1000 sounds more natural for me for the smooth decay and extra of precise harmonics (maybe 2nd harmomics) that make sounds natural some instruments. For me, maybe dx300 sound a little liquid like ultima but i would like some more harmonics.
The body of m15 is also good (NOT precisely correct harmonics but good). Just a lil bit more harmonics in dx300 and it would be natural for me.