If you get the HE400 or any other planar in the future, you may want to consider keeping the schiit stack, as your soundcard doesn't really give much headroom for planars.
Before I go into the differences (I have owned both at once before,) the bass on the HE400 is actually incredibly impressive for its price point. It can go very very low with pretty good quantity. If you've come from a DT990, it may make you think "eh, this isn't that great," but what you're use to is more mid-bass. The HE400 provides sub-bass AND mid-bass in spades...but not as much mid-bass (which most people refer to as quantity) as the DT990 or other dynamic drivers.
The main difference between the two is the fact that the HE400 has a fun (almost V) signature, while the HE400i has a more...linear to dark sound signature. The HE400i has a little more mid-bass and a punchier bass all around, but it isn't the same quality subbass as the HE400. However, the i makes up for it by making a more intimate midrange that is incredibly linear (from the tip of the sub-bass to the ends of the mids) until it hits pretty smooth, but dark, high end. The HE400, on the otherhand, is very....sibilant. The HE400i is also a lot more comfortable, has a better soundstage pre-grill mod, and just looks better in general.
You have to get use to the kind of bass the HifiMan's present, but once you do, you'll start to notice things feel more cinematic as opposed to sounding cinematic, if that makes any sense. You're trading a lot of substance for the actual clarity of that substance. You'll be able to tell what kind of sound the explosion is actually making, and feel it, as opposed to just hearing a thick mass of boom. It's really more of the "realistic" kind of immersion that (as far as I can tell anyway...Might be a coincidental pattern) MLE experiences in the headphones he rates pretty high up there in fun.
If you want to get down to it, there's very few headphones until the Audeze LCD 2.2 and TH600 that outdo the HE400 in overall bass quality. The HE400i is a close second to it in this price-range, but it makes up for it by having a thicker mid-range that makes you feel like you're in that world.