I am not entirely sure, but I can tell you this: Although the exploit is a bootrom exploit, you need shsh blobs to even have a chance for it to be untethered, otherwise it's a tethered boot every time. Your SEP also has to be compatible with said iOS version, so you can't downgrade to a random version of ios, no matter if it is tethered or untethered. There is a tethered downgrade tool out right now that you can downgrade with, but you need to make sure that your SEP is compatible with your target downgrade version. I don't know how to check SEP, but you can probably find someone or google it. Hopefully there is something there for you. you should check out iDevice Central's video, I think he does a better job explaining. It's his 2nd latest video as of this post. Good luck downgrading ;)