brown_bearWhen I demoed beyer in store I found it had a smaller sound stage and a little less clarity, but still a flat response, but that was long ago. I'm also not a fan of exposed wires. Also it's a single sided non-removable cable. The 6XX is a steal at that price and a better headphone imo.
aramkI would expect 990 to have more soundstage and more detail on highs and deeper lows. I think of 990 as different from 6xx, not that 6xx is necessarily better but more pleasant for long listening sessions.
brown_bearDT990 have V shaped sound, with more emphasis on low and high frequency. Consequently, mids are recessed. HD6xx have more emphasis on mids and have some low end, but not as much as DT990. HD6xx has a flatter response, but HD6xx highs are attenuated and can sound veiled. Both headphones benefit from dedicated headphone amps. They are both good and depend on what you are looking for specifically.
brown_bearIf you have interest in mixing music, the Sennheiser 6xx is one of the best deals on the planet. Just a great pair. These DT 990s are in the same class, but the Sennheiser is really at a higher esteem from the opinions I've heard. Both are great. If someone asked me to pick one over the other, I'd pick Sennheiser. I wouldn't fault anyone for buying these, but note the 600 ohm impedance is a different beast. You can get the DT 990s at 250 ohm and they are great too.