I've found that it depends on the Kernel, I have successfully used it on my Tab Pro 8.4 LTE, but wouldn't work with the installed Kernel at that time which had a February date stamp, I changed for a stock Kernel from March and it worked a treat.
If you change the Kernel on you device, I'd stay clear of firmwares that have a June Build date, The TowelRoot thread states the exploit was plugged on June 3rd.
You can keep your current 4.4 build, just download a slightly newer one from another region, extract the Kernel and flash it to your device, then try TowelRoot.
That's what I have done for my device and I have created repacked tar's for a couple of other versions of the Tab Pro, one has worked and I'm waiting to here back from the other 2 variants.
And Yes, Knox has remained at 0x0 on the devices.
If anyone wants to try it out, I'm more than happy to create a package for your device, just send me a PM.
Current Device: Galaxy Note III SM-N9005, Note 8.0 GT-N5100 & Galaxy Camera EK-GC100
Retired but still own: Galaxy Note II, Dell Streak
Used to own:Galaxy Note 10.1 GT-N8000 - Galaxy Tab 10.1 GT-P7500 - Galaxy Note GT-N7000 - - HTC Touch Pro 2 - Samsung Galaxy SII - HTC Desire - MDA Vario III
- HTC Hermes (MDA Vario II) - Raphael - HTC Touch Pro
HTC Wizard (MDA Vario)
- O2 XDA IIi
- HTC Magician (MDA Compact)