I agree with the mid-centric and slightly dry part when compared to the HE400, but still a warm headphone overall. The bass isn't that bad too me, it just lack the punch the HE400 has. Overall I like the HE400 more due to it's image separation ability and it's tilt toward fun.
I haven't spent a whole lot of time with the HE500 yet to really make a bigger comment on them. The HE400 bass to me seems to be the tighter of the two and have a tiny smudge more impact. The HE500 bass to me seems to extend further and lingers a little bit longer. The overall fuller sound of the HE500 might contribute to this, but I do think the HE500 bass is closer sounding to the D2000 with and addition of better texture/layering.
Overall between the HE500 and the HE400, I haven't quite figure out if me spending more money on them is actually worth it, this is me powering them with M/M stack. The HE400 performs so well at it's pricing it making it harder for me to swallow the cost of the HE500 considering I also have to get a more powerful/expensive amp.
another comment I can make is that it is definitely not worth owning both. I will definitely sell one of them eventually.
Don't get the HE-500 because you want a better headphone, get it because you want a different headphone. It might be worth owning both if you want them for the differing listening experience. If you really like the sound of the HE-400, you might think the HE-500 is a bit boring. Then again, you might love it. Won't know until you try. Sorry about your wallet.