Coming back to this, I had the chance to test drive them myself by now. Leaving these impression of my little trained ears for the future lost wanderer:
I can use the Planar without EQ, but I have to dial down the treble on the XS. While applying some EQ lifts the veil (somewhat), I prefer to use the Planars without at present.
The XS is lighter and in general feels barely clamping, more like touching the side of your head. However, I find the headband of the XS producing a hot spot on the top quicker then the Planar. Minimal padding, e.g. a bandana/buff, prevents that leather lump hot spot of the Planar. With my head this is rather optional. I can wear the Planars for 3-4 hours after which I just slightly shift them. Overall they are equally comfortable but with different weaknesses. I prefer the Planar but would appreciate if they would reduce some weight.
Sound stage is wider on the XS. In general these had some initial airy feeling, as if the sound would engulf me, lightly. Those had the potential to let you forget you were wearing headphones, whereas the Planars will rather embrace you physically. This fairy/airy feeling effect did not last / wore down after a brief while so I did not get "surprised" anymore. What remained was a sound that seemed to float above me, hanging in the air / coming from a stage that is elevated. This produced also a feeling of missing...body in the sound. The lows seemed "detached" / not connected from the highs.
I preferred the fuller and more intimate presentation of the Planars. Also they were less sharp when gaming (non-competitive). I wonder if my DX3 Pro+ is a bit thin sounding and therefore not the best pairing for the XS. YMMV as this is totally subjective of course. Overall, to my ears (and with my hardware), both are on the same level, they "simply" provide different ways of serving you the sound.
Here it was not the headphone itself, but the box or rather that foam rubber inlay inside that caused this.