I traded my W1000x for the HE400. The W1000x just ended up being too bright for me. The mids were very good and that was a big reason for getting them. They just pierced my ears and it was hard to listen.
Comparison wise, just totally different signatures, the HE400 sounds a bit smoothies and more liquid. Mids are not as forward but the HE400 has good integrated mids. Planars have this sound where everything is just tight and refined.
Bass the W1000x has good bass but it drops off at the low notes so doesn't sound very complete here, the HE400 has one of the best bass responses I've heard.
Soundstage and image the W1000x is excellent for a closed headphone, it open and airy! Not quite DT770 level but very big.
The HE400 has a good soundstage and accurate imaging. It's superior but it's a hifi open phone so it going to have the advantage.
Mids are close and it's going to depend if you want forward aggressive mids or nicely balanced mids of the planar. I prefer the W1000x here personally. They have some of the best mids I've heard, not quite HD650 but very good. HE400 ave lush, smooth mids, smoother then the W1000x. The HE400 smoothness with the intimate mids of the W1000x
gives you the HD650 mids
Treble I find both good in terms of detail but the W1000x is too bright for me! It makes them really hard to listen to them. I prefer the HE400 for this reason. It's darker, airyer and just nicer to listen too.
I think both are great! If you can deal with the brightness then the W1000x can compete with the heavy hitting planar. The W1000x are overpriced but those woodie cups obviously add do that price a lot.
I think the W1000x is fantastic better then Mad Dog and Shure 1540 ehich are other hifi closed backs. Very underrated headphone.
If I didn't have the HD650 which does everything better, I'd still have the W1000x I believe.