[Witch King]

I love the comfort and bass of my DT 990-600s, however their bright treble and recessed midrange aren't optimal for every genre or recording. I have the FiiO e7/e9 combo to power them. I want to buy new cans that complement their sound. I'd love suggestions for headphones that are very comfortable with similar or better lows, more midrange, and tamer highs.

Headphones I've heard: I tried the HD 800 briefly in a Tokyo store, yet all I remember is their superb comfort, but not the sound. I didn't care much for the HD 600s/650s in terms of comfort, and their sound made me sleepy. The AT M50s were solid all-rounders, but they too had recessed mids. The AD700s were comfortable, but too bright and little bass. Hopefully this may give an idea of what my ears like.

 

[Claritas]

  I love the comfort and bass of my DT 990-600s, however their bright treble and recessed midrange aren't optimal for every genre or recording. I have the FiiO e7/e9 combo to power them. I want to buy new cans that complement their sound. I'd love suggestions for headphones that are very comfortable with similar or better lows, more midrange, and tamer highs.

 
It sounds as if you're looking for a planar. The problem is that most of them are uncomfortable, with the exception of Fostex mods such as ZMF, MrSpeakers, &c. So you might want a Fostex mod or to wait till the new Hifimans come out later this year.

 

[KG Jag]

Here's the problem.  When you jump to tiers of cans above the DT 990, you start to run into a mismatch with you Fiio equipment.  You should strongly consider getting something better to go with $400 to $600 cans--even if you you don't technically need more power (for at least some options).
 
Mad & Alpha Dogs should be on your list to check out.  I would add the HE-400 & 500 (the second of which clearly requires a better amp to get what you pay for), although they may be too heavy or otherwise less than super comfortable for some--and perhaps you because did not like the comfort of the HD 600 & 650.