Thanks, earfonia. You're awesome! I'm going to have to recable my headphones too now, once I get my hands on some starquad.
I detected the crosstalk on my M50s too, and also in a bunch of my cheaper headphones. But no crosstalk on my Grado SR60 (the old version). And from having replaced the 3.5mm jack in the past, I know the SR60 has four wires inside the cable, though not as nicely done as the starquad, and not shielded either.
That brings me to my (n00b) question. Does combining the ground wires with the starquad shielding at the jack end do anything? My Shure E2Cs use a shielded cable, but with only 2 wires inside, using the shield as the common ground path. And they have crosstalk.