Yup, the XM3 sounds fantastic when cabled with my Dragonfly Red—the bass is tamed is bit, the mids are a little more forward, the treble is slightly crisper, and the noise floor is imperceptible. I really like that they can be driven while powered off, by comparison to the PX. In fact, the PX isn’t even compatible with the Dragonfly Red IMO; there’s something about how sensitive the aux input is, combined with how its own DSP is always running, that causes an unacceptable noise floor with noticeable clicking. The only cabled benefit the PX has over the XM3 is that the PX can play audio via USB C, but I’d still prefer to use my DFR if I go to the trouble of plugging in.
And after some more time of comparing the two (wirelessly at least), I know what you mean—the PX is a midrange monster, and combined with its (surprisingly) wide soundstage, it ends up sounding a bit boxy. The XM3, by comparison, has a much more intimate presentation, with less emphasis on the mids and more on the lows and highs. In fact, the XM3 sounds a lot more like the P7 Wireless than the PX.
Completely agree about comfort between the two. The PX becomes acceptable over time, as the pads become softer. But the XM3 is wonderful out of of the box—it doesn’t really clamp, and the reduction in weight does a lot for helping these just disappear when you’re wearing them.