the xs and the sundara in my opinion are very different in sonic signature, the sundara seems more focused on the fresher mids while the xs seems further back on this but is more decisive more separation and body, in my opinion if the xs would have a mid little ahead how the sundara would be perfect.
My take is the Sundara has a bass light profile. It's very much missing not only impact but db under 50 Hz, and its not really convincing under 100 Hz. It's also got a peaky treble in the 6-9k range.
The XS also lacks impact under 50 Hz, but it holds on better, and takes EQ well to fix the db if not the impact. Trying to lift the Sundara to flat or a slight bass plus sound from 20-30 Hz results in just limpness. The XS has a peak at 8k and 12k. The 12k one doesn't bother these old ears. The 8k peak is a problem, but like most HFM cans including the Sundara control that 6-10k area you still have that sense of life and transparency that is a hallmark of HFM planars.
Also the soundstage is much larger on the XS. Not specific like the 800S, but similar to the HEX v2, Arya, and Ananda. I have not heard the Drop XX in almost 4 years but the tuning on the XS is better than that, XS also less peaky than the Ananda (pre stealth), and well less edgy than the Arya (pre stealth). It's finely tuned for everday use, and its fine on equipment you would tend to pair with it, like various low->mid Schiit gear. Streaming TIDAL (1st unfold), UAPP parametric, thru a Gumby A1 and a Rag 1 is the best I've heard the Sundara, but, the lower bass could not be massaged, and that's likely to be a better set up than most would use with it.
The XS is good enough that if you just listened to it on good or better gear, you'd habituate to it, and consider it quite accurate and pleasing - even with no EQ. The Sundara just can't handle massed strings and female vocals in a way that I can relax with in the now or over time, for compressed rock/pop - its not as evident.
