Just received my DT-1990s today, so here's a summary of my first impressions, compared to the DT-770s I've been using for the past 4+ months.
Build: The aesthetics and build quality of the DT-1990s is definitely a step up from the 770s in the way they look and feel. The clamp force out of the box is less than that of the 770s (when new), and the ear pads fit perfect around my ears. While the 1990s are actually heavier than the 770s, they are actually more comfortable, probably due to the improved headband and less clamping force.
Sound: In a word, outstanding! The clarity of the Tesla driver shines through right from the start, and I assume that will only get better as they break in. The lows are punchy, tight, and well defined. I thought I might need to kick in the XBass feature on my amp, but no, it's absolutely not needed with these drivers. The mids seem to be a bit more forward than the 770s, but I'll need to listen to a few more tracks to be sure, but like the bass, they're very well defined. The highs are very present and sound crystalline in the 1990s, where they're a bit more metallic with the 770s. Overall, I'm hearing more of everything with the 1990s, especially the subtle harmonics that often get lost in the mix.
First day, first hours, and very happy with my choice to get the DT-1990s!