I used to have the DT-990 250 ohms and sold them. The DT-990 have way too much tremble IME. Painfully, fatiguingly, and unstandably so. They have more tremble than bass for sure. Here are some measurements that correlate well with what I've heard:
Hearing Tomita's Clair de Lune out of my DT-990 was an ear numbing experience... I don't think the M50 are nearly as emphasized in the tremble as the DT-990. Here is a plot of the frequency response of the M50s:
With all of their flaws, I prefer any of the Sennheiser HD5x8 products over the DT-990. Used, an HD558 can go for $100 to $110 depending on the market. They are $150 new from Amazon now. The HD558 is also easier to drive from most DAPs relative to the DT-990 250 ohm.
There are better headphones of course, but they will likely go north of $200.
Also, IMO a Paradox (professionally modded T50RP), will destroy a DT-990, HD5x8, and M50s. Not even funny. But a custom product like that will justifiably command a higher price. You would likely also need a dedicated amplifier for a Paradox.
Again, regarding my now gone DT-990 250 ohm, to be blunt, my humble-fi set of KSC-75 mini pancakes seem polite and gentle to my ears relative to the likes of the my now gone DT-990 tremble uber-happy cans. Cannot recommend the DT-990 unless planning to run them out of a high-frequency rolled off and gentle amp, or if shamelessly using an equalizer to tame them.
Edited by ultrabike - 1/28/13 at 4:46pm