I would kept X2 too to be honest but 770 was my own unit so it wasn't returnable like the X2. I returned X2 and bought the 1990. DT 1990 price is really good in EU. It's one of the best buy for the price in my opinion however it's very expensive in US where it reaches Elex, Ananda, LCD 2C tier of headphones (i have Ananda but haven't heard others yet) so the competition is fierce. It's Sundara competitor in EU. Ananda competitor in US.I recently sold my DT770 and kept the X2 because they are pretty similar. I love the soundstage and build on the X2 more, so I kept those, but I do miss that DT770. If I could have replaced the cable, they might have stayed with me. I've actually been curious about the DT1990 for a while, but never could find a good price on it. It's essentially everything I wanted out of the 770, so now I'm very interested.
The HD800S is amazing. It's tuned bright, but after your brain adjusts to it, the sound is simply so good. Everything sounds uncannily outside of your head and clear and precise. Really phenomenal, IMO. But, they can be an acquired taste. Even on the best gear, they can be too bright for some people. On mediocre gear, they can sound harsh and thin. If that's the sound you're after and you can provide them with a suitable amp, they really are killer.
1990 doesn't have the soundstage of the X2 or even 770. The soundstage is still good though. It's not an issue. It's just not gonna satisfy if you want big soundstage. They are forward and aggressive, very aggressive. Pounding bass, energizing treble, accurate(no dips and peaks) warm mids (elevated treble can skew mids though). It's warm and bright at the same time. Treble peak is too much. Very fast, very dynamic, resolving. It can not be a main headphone but it would be great as an alternative to reach out when you are in the mood. Even chill, sleepy music sound energizing on that headphone lol. Overly stimulating.
I have the Zen Can right now with 4.4mm balanced it can output almost 1w at 300ohm which is very impressive. There is also the xbass button in case of bass roll off. My amp looks good for the HD800S on paper. I feel like this is the most logical high end option for my taste.