Here's my explanations with whom I placed. Typically I don't feel DOA has a blatant SS tier cause of the system in place. However, considering that I use MMCafe for tier list making with Tekken I made an exception.
My SS's are characters with really strong neutral. Really good Crushes or evasive tools which can torture match-ups regardless. They work really well with the stun-game and a high amount of knowledge is needed to beat these characters.
My S are characters are similar with the SS's but there's something lacking. Whether it's the high execution (Akira/Pai) or some match-ups that hurt their placement (Rig) these characters are really good but there's one thing missing to put them with those SS's.