How to reset completely to stock?

So Im no noob to android nor the rom/rooting crap. But i come from nexus devices and we all know how super simple those are to use and return to stock. I dont understand the HTC crap. I bought this m9 and thought it was going to be dev friendly like nexus's are but I was wrong. This is what I did. Unlocked bootloader, flashed TWRP (and before I made the backup, I set it to r/w) then i made a stock backup that was set on R/W and did some other things like flashing CM etc.. My question is, if i wanted to return to stock and be able to receive OTAs, what must I do. Would downloading a stock nandroid from here be enough or must I run an RUU(which i dont understand). Also, I still have S-ON and have not modified that. If read some guides but Im still confused. Nothing like a nexus. Any help would be appreciated. Thanks (tmobile USA)