Please let us know how it goes.
My 8.9 hdx is also soft bricked.
I did a reset in safe strap to format data cache and SYSTEM but now when I reboot it sits at the white KINDLE FIRE screen.
I CAN get to the stock recovery screen by holding the volume + and power...or press power then hold vol +. All that does it let me reset the device but I guess since I formatted the system partition there is nothing for it to boot into.
I can't seem to get fastboot to work. I can't get into fastboot.
I am using win7 but the device shows up as MPT device with a yellow!
I did try linux Mint17 and it shows up as a google device. I tried adb devices but it listed nothing.
Please let me know what you do to fix yours.
There has to be a way to reflash this this thing