Interesting since I was also considering the D1 before purchasing the Fulla 2. I ultimately chose the Fulla 2 since it requires no extra adapters for my headphones and speakers. The cheaper price was another added bonus.
I bought the Fulla 2 looking for more power since Audioengine is about as vague as possible on the mW output of the D1 and if I was really into a track I could max the D1 with my HE-400i's. I'm finding the Fulla 2 can get louder as expected but the bass...
The D1 immediately impressed me by how full the sound was on first listen. The Fulla 2 on the other hand is much brighter. This isn't a "bass by the numbers" difference like bass'y cans vs treble'y cans. It's every appreciable meaning of the word bass. Speed, extension, control, presence, decay, sub-bass...
After all of that I'm still not saying the Fulla 2 is bad ha ha. Once I listen more though I'm the type to call "better" better so we shall see how they compete when my ears have adjusted to what the Fulla 2 is. I can't judge it fairly yet because my ears are in "competition mode" and I'm willing to acknowledge that.