I have both DX300 with Amp11mk1 and Amp11mk2 and M8. My Legend X is currently being refit so can't side by side compare right now. I overall agree with
@Wildcatsare1 take on that M8 on that it sounds more natural. I do however primarily listen to pop, electronic, edm, rap, rnb, vocals, sometimes instrumentals and rock. I haven't heard other DAPs higher end than M8 and DX300, but what makes M8 unique to my ears is it is able to have this warm, organic, and smooth sound without sacrificing much in terms of resolution. M8 doesn't sound like I just used software EQ to warm up the sound which I personally don't like the sound of software EQ'd warmth. Maybe it's better to say the low end sounds more full or meaty on the M8? Another range to highlight are the upper-mids/mids. I find that the M8 is able to smoothly push forward the vocals without being aggressive. On DX300, the vocals are forward but not as smooth as M8 so some people find upper mids overemphasized when paired with an IEM that emphasizes upper mids already. DX300 I also find to have noticeably less resolution compared to M8. With Amp11Mk1 on DX300, there is a more noticeable mid-bass bump and not as much sub bass, but on M8 there is both a mid-bass and sub-bass bump.
In terms of cons, maybe M8 lacks a bit of speed that is needed for some electronic music in comparison. To some LX might sound a tad too warm on M8, but I didn't think so after I gave it some time for my ears to adjust to the sound. I can't confidently say that the LX and M8 is an incredible pairing, but I do think it's a good pairing. I can elaborate more later assuming I don't sell the DX300 by the time I get my refits back. Since I don't have them side-by-side, it is harder to properly compare in the way you would like.
Edit: Where I noticed a bigger difference between the M8 and DX300 was with the MEST Mk2 (when I had it). MEST Mk2 is a bit incoherent sounding to me on its own, but the M8 was able to smooth it out throughout the frequency range better than the DX300 and I noticed better resolution there as well.