CukedaddyBeen auditioning them side by side for a few days now, with Stax SR-007 and Hifiman HE-1000 headphones, through Stax SRM007t and Woo Audio Fireflies WA7 tube amps respectively. Playing FLAC files, via tidal.com.
With either headphone, I give the clear nod to the M8. Sub-bass is just a little more powerful and tighter, and the overall sound is more "holistic" to me, at least with small lineups like most jazz and blues bands (the jury's still out on densely packed rock and orchestral works).
I have a bunch of Schiit gear (Fulla, Magni 2, Modi 2, Bifrost Multibit, Lyr 2) and am predisposed to liking the brand's, well, schiit. The U.S.-built equipment may well be constructed better than its Chinese competition (I'm just guessing, admittedly), but from a purely sonic POV, I easily prefer the M8 over the Modi 2.
By the way, I'm playing my M8 without the P1 power supply (using the M8 wall wart for now, because I have some incompatibility issue that I'm trying to work out with the seller of the P1). But I suspect that using the P1, once I get it to work, might improve things even more in the M8's favor.