I think the replacement to for the Amazon Fire TV box is here
It puts out 60 Hz 4k video and has audio passthrough! Comes bundled with Plex too.
Nvidia Shield Console
Specs:
Processor NVIDIA® Tegra® X1 processor with 256-core Maxwell™ GPU with 3GB RAM
Video Features 4K Ultra-HD Ready with 4K playback and capture up to 60 fps (VP9, H265, H264)
Audio 7.1 and 5.1 surround sound pass through over HDMI
High-resolution audio playback up to 24-bit/192kHz over HDMI and USB
High-resolution audio upsample to 24-bit/192hHz over USB
Storage 16 GB
Wireless 802.11ac 2x2 MIMO 2.4 GHz and 5 GHz Wi-Fi
Bluetooth 4.1/BLE
Interfaces Gigabit Ethernet
HDMI 2.0
Two USB 3.0 (Type A)
Micro-USB 2.0
MicroSD slot (supports 128GB cards)
IR Receiver (compatible with Logitech Harmony)
Gaming Features NVIDIA GRID™ streaming service
NVIDIA GameStream™
SW Updates SHIELD software upgrades directly from NVIDIA
Power 40W power adapter
OS Android TV™, Google Cast™ Ready
Bundled Apps PLEX
In the box NVIDIA SHIELD
NVIDIA SHIELD controller
HDMI cable (High Speed), USB cable (Micro-USB to USB)
Power adapter (Includes plugs for North America, Europe, UK)
Requirements TV with HDMI input, Internet access
Options SHIELD controller, SHIELD remove, SHIELD stand
Weight and Size Weight: 23oz / 654g
Height: 5.1in / 130mm
Width: 8.3in / 210mm
Depth: 1.0in / 25mm