I'm still leaning towards the HiFiMAN HE-400 at the moment. From what I've taken away from reading the forums, is that the DT 990s have similar characteristics to the DT 880s with more bass. But the HE-400s characteristics are completely different. Though I have read some people mentioning the DT 990s have more bass than the HE-400s.
I have it narrowed down to these two headphones currently (as always this could change in seconds ), now I just need to decide if I want to sacrifice some bass for a completely different headphone.