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