What would you recommend as a clear upgrade?
Fidelio X2 supposedly has smoothed out the treble even more than on the X1, and tightened the balancon it's bass so much that midrange shines slightly more. Therefore though it wouldn't be a huge upgrade, I'd recommend the X2 as a clearer upgrade than the X1 or DT990.
The HE-400 would also be a clear upgrade in many facets, better bass, fairly engaging mids, mostly smooth mids.
The HD600 would be an upgrade in nearly ever facet, though electronic music sounds slightly too slow in their decay.
It kind of depends on what you out of your headphones at this point, and things are only going to get more complex if you keep trying to upgrade. I'm still searching for the upgrade answers at this particular point in my headphone career. I've nearly exhausted all of the easy options without spending too much money, but I'm fucked.