Although they have advertised 250 Ohms impedance you average smartphone can drive them pretty well to painfully strong sound pressure level. The caveat however is that if you pair them with a simple amplifier you will unleash the beast <1KHz. I was pleasantly surprised when I first tried them on my phone, then thrilled on my audio interface Komplete 2, finally the HA4 "KOMPLETED" the circle of audio pleasure. I compared them directly to Custom One Pros, Marshall Major and K-701 all four driven from the same source - Komplete 2 + HA4. I equalised the volume to the best of my perception and the DT 990s wiped the the floor with the competition.
Sound: 5 stars - NO DOUBT ABOUT! (Metal, rock, classical music listener here)
Comfort: 5 stars - Pretty fluffy ear cups. I wear glasses and my deal breaker is that the headphones do not press my ears too hard against the frame of the glasses. I do however fear that during summer the ear cups will be a bit too warm.
Quality: 4 stars - Overall pretty solid BQ. I do not like the way that metal brackets on both sides are attached to the head band (Mechatronics engineer's inner voice says - FLIMSAAY!). Luckily ALL parts can be ordered as spares from beyer when they break. I am also not of fan of the fixed cable... that should've been a detachable 3.5mm jack like many other less expensive headphones. The cable itself is a bit of overkill sized gauge telephone coil. It is massively thick and heavy and absolutely not portable. It's not a big problem for stationary setups but please take in into account for portable intentions - they are not.