I used to have a 2016 pair of HE400i’s, and in hindsight, it’s probably one of the most atypical pair of planar headphones that I have listened to - and the 2020 revision didn’t do much to remedy this. They have a noticeable drop off in the mid to sub bass region with the stock pads, which give them a noticeably “thin” sound signature, and if you’re used to sound that is not this anemic, then this may explain why you’re left wanting more.
With that said, a more powerful amp could offer the headroom to apply some EQ to add back some bass. Assuming your audio interface has some power left over, you can try this now for free, which you can start with this:
https://www.reddit.com/r/oratory1990/wiki/index/faq
You can then find your headphones from the listing here:
https://www.reddit.com/r/oratory1990/wiki/index/list_of_presets
Since you’re on a Mac, I can definitely recommend checking out Rogue Amoeba’s Soundsource:
https://rogueamoeba.com/soundsource/
For full transparency, I haven’t felt the need to use any EQ lately, so have not touched it for a long while, however, when I was trying to replicate the sound performance differences between my Macs and Windows machines, Soundsource ended up being the easiest path forward - even though it does cost money. With that said, I purchased my license in summer of 2020, and am still getting regular updates today, which is nothing to sneeze at in this day and age. They also do offer a free trial period, so no better way then to try for yourself, especially if you’re running into similar hurdles as I did.
Lol, yeah it is a pain in the ass to set up at first, but once it’s done, you should be able to essentially set it and forget it. However, to that point, audiophiles are known to endure endless suffering to eek out subjectively meager gains… :)