I`m definitely in the Apple TV camp. I connect to the HD621 via HDMI and the music sounds amazing. I had some problems experimenting with streaming using Airport Express and would not recommend it based on my experience.

Meridian: 861v4 + ID40, HD621, Ultra DAC, 857, 558.2
Speakers: B&W 802D, B&W HTM2D, 2-15" custom subwoofers, B&W LM1
Other: Oppo BDP-83, Mac Mini Roon Core.