Pros: Prominent and detailed highs, very engaging, extremely comfortable, very open with wide soundstage and a fast-paced bass
Cons: Harsh treble (to some ears), slightly recessed mids, and rustling drivers when combined with heavy bass tracks on high volume
I picked up the Beyerdynamics as my second real hi-fi phones to replace my Sennheisers HD598 in order to get something bassy, yet with an open construction. While they were a huge improvement to my previous phones, they did not reach my expectation in the bass department, and I will explain why later on.
Treble: First off I should mention the initial sound signature of these headphones. They're very bright! You might have read about its treble, and it is true, they are truly trebly phones. But the treble is not at all what some people exaggerate it to be. Yes, the treble is noticable, and is perhaps the first thing that will strike you when you first listen to them. But the treble is detailed and deep and not sibilant, and it serves really well with some pop tracks and electronic music. The treble is also very engaging, it brings life into the highs and brings out the cymbals, but they also made female voices stand out before the instruments, as well, which is a positive thing when listening to pop.
Soundstage: It's wide, and great! As these phones are open, you might expect them to perform well in the soundstage department. With the wide soundstage of the DT990s the treble does not strike too close and too engaing to your ears. They're at a distance, but well detailed and refined. Making them even more ideal for electronic music. For electronic music I chose Susumu Hirasawa, where soundstage can make an impactful role. On some tracks the voice is supposed to go as an echo among the instruments in a distance. Though it is a male voice it becomes slightly more apparent than the high tone instrumental tunes, and it creates a life-like sound stage with a good separation of instruments on some of the tracks from Fleetwood Mac - which in comparison does a better job on the DT990 than the HD650, which is more close-sounding than the Beyers, but with less prominent highs. But again, sometimes the male vocals are less prominent and less refined, making it sometimes overwhelmed by the other instruments. Of course, this was quite rare with Susumu Hirasawa as his voice is generally fairly bright on most tracks. But it did not work well for me on male vocals in operas and some orchestral music.
Bass: Yes, they have bass, which many other head-fiers and pointed out. But some may exaggerate a bit here as well as with the treble. For an open construction these cans have some bass that amounts for quite a bit, but it is also detailed and far from muddy or overwhelming. It's fast-paced and you will notice some punch to it on some tracks, especially to some dupstep tracks--but know that these cans are not exactly ideal for Dupstep or Hip Hop. They are generally bright cans, and while they do have a fast and detailed bass, it is not in the spectrum of being plentiful enough to be considered "Bassphones." However, to some tracks the bass is quite noticable and manages well with emphasizing the lower frequencies: such as the beginning of Massive Attack - Angel, as well as the punchiness of Morgan Page - In the Air.
More importantly: If you listen to some bass-heavy tracks, such as the start of Morgans Page "In the Air" (especially if you are using any equalizer to enhance the lower frequencies) then at a certain volume level one or both of the drivers will rustle. At first I thought I had a faulty product, but investigating on the forums led me to several other cases where people suffered from rustle on bassy tracks with high volume. Amplification might help in this case, and you might be able to raise the bar of the volume level before reaching the rustle, but only slightly if so.
Comfort and Design: This, alongside the soundstage, is my favorite thing about these cans. When I first took them out they looked really robust and uncomfortable. But as soon as I took them up I realized how light they were and how extremely soft and comfortable the earpads on the thing were. Taking them on was a breeze, and the headband padding was also very comfortable - it felt as if you weren't wearing anything on your head! Again, the design, as mentioned before, looks very robust and industrial; especially how the cables go slightly bent/rounded above the earpads - and this sort of design does signify just what type of sound signature you might expect: bright and detailed with a wide soundstage (the width of the earpads). To me, the design does not necessarily strike as a pair of bassy headphones, and to some extent that is true, but it still does have a slightly more impactful bass compared to the HD598 and the HD650; but it is often associated with the treble, making drumkicks and such sound brisk and bright but still quite punchy.
Comparison: I'm not even going to compare it to the HD598, the bass, detail and comfort is far more superior on the DT990s. But I could compare it to other phones in proximity of its price-range and sound signature; and of course, with headphones that I have tested out and own(ed). To the HD650, I prefer the DT990 on most electronic music without vocals (or female vocals if included), for the electronic music I prefer to listen to the male vocal is slightly brighter than most other male vocals, making this artist more ideal for the Beyers. The DT990 was also way more engaging and more open than the HD650. I love the HD650s equally as much, but for electronic music, pop, and some classical music I preferred the DT990 where the soundstage made more impact in addition to the well prominent and detailed highs, making electronic music feel electronic. The Sennheisers are more musical/warm, and are less engaging and bear less emphasis on the highs than the Beyers, but I did feel more bass quantity on the HD650 on some tracks. However, I did feel slightly more detail on the DT990, while the HD650 were a little bit more muddy in comparison to dupstep and songs like Massive Attack - Angel and Morgan Page - In the Air.
As for the Ultrasone Pro 900, the DT990 600Ohm is quite similar in terms of sound, but some areas should be considered to be very differentiating. For example the bass on the Ultrasones is ALOT better in both quantity and quality - but the highs are far too bright on the Ultrasones and softer on the DT990s. Also the soundstage felt more natural on the DT990s - most likely cause the Beyers are open, and the Ultrasones are closed. To my ears, the Ultrasones had wider soundstage but it was unnatural in comparison to the Beyers - this could be due to the holographic S-Logic of the Ultrasones.
Conclusion: For the price range, the DT990 600Ohm comes with a lot of value - especially for movies and games. But the rustling drivers, the slightly recessed mids, and the high amount of treble are things to consider before purchasing these cans. In all honesty, despite its cons, these are really quite pleasent headphones to have - if you can get them cheaply. And they work even better with OTL tube amps. When I paired them to the LDMKIII with the Voshkod tubes, some of the 'harshness' was reduced. Or rather, the treble felt less protruding, but the clearness and the detail of the highs was still intact. If you're listening to electronic music and pop, then I can warmly recommend these headphones.