I think that is a Sense 5.5 stock ROM, not sure whether it's rooted or not (I've not found a development thread for that, just a few sites that mention it, so I'm having to guess a bit as to what it is). If it's not rooted you'll need to do that before the S-Off tool can work (rooting describes the state of the ROM, so a phone can be unlocked and have a custom recovery but also have an unrooted ROM, which is why I'm raising this question here).
You could try installing this kernel: [KERNEL] [April 21] Bulletproof - xda-developers
. There are versions for different Sense/Android versions listed in the first post. I'd guess you want the Sense 5.5/Android 4.4 version. If there is a problem, reflashing your ROM over it will restore the original kernel. Rooted plus that kernel should give rumrunner a chance.