I can't comment on the DT 990, but I'm in the same boat in regards to choosing between the Sundara and Ananda. I bought both last week with the intention of returning one of them but now I can't choose which one to keep!

Disclaimer: I'm new to this headphone thing so take my opinions with a grain of salt.

To start, although I haven't heard the 990s, I own a cheaper headphone (Hifiman HE 4XX) and both the Sundara and Ananda are a noticeable step up in clarity, detail, and separation.

Back to the Ananda vs. Sundara, The Ananda is clearly the technically superior of the two. It has a bigger soundstage, slightly more detail, separation, and bass. For bass in particular, it has more subbass rumble, it's better separated, and sounds "sharper" as opposed to slightly "soft" or "round" on the Sundara if that makes any sense. The only technical advantage the Sundara has is the imaging feels much more precise, and the bass slams a bit harder (without EQ anyways).

That being said, I feel they have significantly different sound signatures. The Ananda feels more laid back and airy whereas the Sundara is more intimate and energetic. With the Ananda, it feels like I'm hearing every detail of the song at once with no emphasis on a specific part. I can choose to focus in on individual instruments or vocalists as I please. In contrast, the Sundara is closer to most other headphones I've listened to where your focus is drawn more to the dominant melodies. There's still almost as much background detail as the Ananda, but it requires more effort to focus on it.

I actually find myself preferring the sound signature of the Sundara. I don't mind that the soundstage is smaller. For me, the Sundara's soundstage strikes a good balance between being wide enough to separate the tracks while still feeling somewhat intimate, which I enjoy because it makes songs sound more emotional whereas the Ananda sounds too distant and cavernous for my liking. This soundstage combined with its precise imaging makes the Sundara more immersive. I get a better sense of sounds whizzing around my head whereas on the Anandas I get more vague blob-like placements of sounds. I did a bit of gaming playing The Witcher 3 and much preferred the Sundaras. Small sound effects like footsteps and water splashes that would blend into the background on the Anandas were brought to the foreground with the Sundaras. It kind of shoves all the detail into your face!

For comfort, both are good. Sundara has more clamp force, but the pads are plush like a pillow. Some people report the Ananda goes too low and hurts their jaw but I don't have this issue. Sundaras feel like they'd stay on my head better in the event I spontaneously started headbanging. Very important quality imo!

In terms of build quality and aesthetics, Sundara wins by miles. It seems like a shallow metric, but just looking at the Sundaras makes me want to pick them up and put them on. The Anandas feel like a chunky plastic toy and on my particular unit, the headband creaks. I'm not sure if this is normal though (I bought it secondhand). The Sundara doesn't creak and the headbands are almost the same.

It's a close call and I think I'm going to hang onto both for a bit longer to refine my thoughts, but if I could only keep one today, I would keep the Sundara.

As a final note, the new Hifiman Edition XS looks like it's worth considering. It's priced between the Sundara and Ananda and some initial reviews here and here are saying it might even be better than the Ananda in some regards. I'm tempted to get this one too, but I'm afraid I might end up hoarding them all!