I feel like part of it is that it's a very un-congested presentation, so bass seems to slam harder as it jumps out of a blacker background and from a clearly defined area in space. I've heard other setups where you need to focus to hear the bass lines, as it runs up against other elements in the music.
The first time I really heard the HD800's with great bass slam was on Purrin's PS Audio PWD --> Balancing Act setup at the last Bay Area meet. I don't think anyone could walk away from that set up and think the HD800 was bass-light, or thin, or clinical. I use James Blake's "There's a Limit to Your Love" as one of my bass test tracks, and that song really hits on that system. It's not like the bass was overwhelming or boosted either, it was still supremely balanced and natural. It's just that the bass was so dynamic, and could seemingly jump from one to ten instantly from out of nowhere if the music called for it.
I don't have $8K to drop on DAC+amp (from what I understand, he's using $1K+ in tubes), but that has been my unicorn dynamics system since. Sort of an "endpoint" in mind.
It's been one week since I got the Gungnir, but I feel like my system now shares a lot of the same qualities as that reference (from memory). Sure, it may not match the resolution, or micro-detail, or dynamic shifts of the PWD-->BA -- but it ticks a lot of the same boxes. Musical, involving, analog and natural, very smooth treble, lots of warmth and atmosphere.
One thing it does match the PWD --> BA system in is its ability to make me NOT want to listen critically. It has a way of just sucking you into the music. Seriously, the other night I just got my Magnums back from a long term loan from another member. I plugged them in just to check if they were still in working order, and ended up sucked into a 4-hour session with them.