Almost bought the Jotunheim 2. Also have a hybrid headphone amp at the shop for servicing that I may try with Sundara later, but that will require a DAC upgrade (which I am trying to (economically) avoid).
My first use of power/integrated amplifiers to drive headphones came about when I had the (original) HE-6, a tough load, and during a time when comparatively few dedicated headphone amplifiers (at least those I could, or wanted to, afford) existed.
After a lengthy (and costly) term, I had gotten away from headphone listening and gone to nearfield monitors (see signature). Consequently, I ended up with three DAC/amps at my disposal after experimentation: the Wadia 151, Wyred4Sound mINT and bel canto C5i, all Class D. The W4S and bel canto have headphone outputs. The W4S's headphone out was not sufficient for the Sundara. (Haven't tried the headphone out--or the speaker outs--from the C5i yet.)
Have also used the W4S to drive the Sundara directly from the speaker taps. Very good, too, but not as good to my ears as the 151 (which has a better DAC and Wadia's typically superior digital expertise/implementation).
The aforementioned DAC/amps employ (IMHO) well-executed Class D, and I enjoy the speed, drive, detail and "cleanliness"/orderliness they convey. That said, however, I think I understand your perception/association between Class D and "compressed" sound.
The Sundara have served as my reintroduction to headphone listening, and come as a pleasant surprise: a true "benchmark" both for HiFiMan, and for planars in general.