I had the very same problem. I connected my phone to the amplifier and the BT connection worked fine (I played some music). When I started the iPhone app, the app just froze. I tried that several times. Then i downloaded a new copy of the app... same problem. Then I powered the amp down. After that it was completely dead. So, I tried doing a few factory resets. No luck. Then I downloaded the USB firmware updater to my Mac. I ran that and the window was black. No way to force a firmware update. I believe the problem is that the firmware is corrupted and it cannot be reset with the normal factory reset (master volume knob and tone button pressed while powering up).
So, I'm lost. I've submitted a request and haven't heard anything from Line 6 yet.