The DT 990 had a strong "V" shaped sound signature. Generally speaking cans move closer to neutral than that sound signature as they move up their line. Most all of us would have to search for the initial release information for each of these cans and/or write the company who makes/made them. That is something you can do as easily as we can.
There are a number of reason that some people select to go upline with Fiio and other brands. Quality of sound reproduction and the types of cans the device will drive properly just get the list started.
I'm not an engineer. However, it seems a bit strange to me to assume that producing a certain level of volume tells you that the can is also producing the amount of bass, clarity, imaging, etc. that it is designed to deliver.
That's incorrect. Although true with regard to some headphones, there are a great many who can be driven to even loud volumes but do not deliver the sound for which they are designed and capable unless they are properly amped. Note: I am not talking about low impedance cans with relatively high sensitivity ratings that can be easily powered by almost any device.