Edition XS uses two newer Hifiman technologies to improve the sound quality: Stealth magnets and Supernano diaphragms. Stealth magnets are curved on the outside surface to improve airflow and reduce back-wave interference - this improves clarity and makes for a more spacious soundstage.   
 
Hifiman Edition XS super-nano diaphragms are 75% thinner than the original, with the thinner & lighter material improving clarity further. A low sensitivity means these still prefer a decently powerful amp and can produce very dynamic sound and excellent separation of instruments. Bass response is stronger with more power and depth than the Sundara.
Edition XS ear pads are thick, angled for comfort around the ear with tons of space for your ears and are very breathable. The headband has thick padding with no hot spots. Cable looks basic but feels durable, does not tangle or transmit noise - you do not notice it, which is what you want from the cable!
Edition XS sensitivity rating is low, so feed these a bit of power in order to experience their max performance.
 
Hifiman Edition XS review : Headfonics.com review, 8.9/10:  I could easily wear these all day long.... The HIFIMAN Edition XS has a large soundstage, precise placement, and imaging, an almost true tonality only rivaled by its much higher tier cousins plus the bass has finally arrived at the HIFIMAN house sound signature.
 
For a matching dac/amp... Fiio K11 R2R or Aune X1sGT pair great!, FYI. 
 
For a balanced cable, Meze 4.4mm BAL, $79.
Hifiman Edition XS vs Sundara: Bass response, especially sub-bass, has better power, is more present and accurate with the XS. XS has an even larger feel to the sound. XS has more spacious ear cups.
 
 
Specifications
 
Sensitivity:  92db
Impedance: 18 ohm
Frequency Response: 8-50,000 Hz
Weight: 405g
Includes: dual 3.5mm - 3.5mm 1.2m cable, 6.35mm adaptor
Warranty 1 year