Hi, I used a wired XBOX 360 controller successfully with the Steam Link app. I've purchased a new XBONE controller and when I connect it to the tv and hit the x-button it instantly shuts off my tv, lights up, vibrates and the tv's stuck in boot. TV is a Bravia X940E btw.
You have bluetooth connectivity and your Xbox One controller(assuming its the wireless type) will connect through bluetooth. Try connecting it through Bluetooth.
I'm under the impression that it is through a wired connection.
On a different note it can be due to the key layout being misconfigured, on my Android TV the triggers were swapped and I had to recalibrate/assign them in Steam Link app.
I'm guessing that "X" is assigned to the power button? More information would be great to problem solve this.
Connect it to a powered hub