The impact qualities are overall better than my Sundara I'd say(both EQ'd), which many find very good. It's probably all frequencies below 100hz have good slam to my preference, not just sub bass (20-50hz). The Sundara I think edges it out above the 100hz range, but not by a lot (again, I think this solely has to do with the fact that your ear is a bit closer to the driver with the Sundara). The Sundara starts to roll off at around 50hz, so this is where it's a no contest.
I had my brother try them for a song, and the first words that came out of his mouth were 'that's a basshead headphone'. So we MUST be talking about different things when we say 'slam/impact'. I'm actually really curious how you guys define it. What are your EQ settings and what songs/tracks are you using to test this?
Let's exclude very expensive headphones like the LCD-4, as I haven't tried it and many people in the market for the Edition XS won't be also considering a 4k headphone.
Edit:
Here's a very recent review that agrees with my take:
https://www.mmorpg.com/hardware-reviews/hifiman-edition-xs-review-2000124181
"These aren't a bass-head headphone but they can absolutely be EQ'd to be. "
So, in what way are some like myself experiencing something very different than others in here? Let's get to the bottom of it.