If you dont supply the default scheduler, it want do any scheduling, therefore wont create any threads.
I would hazzard to guess that you dont want to be using Subjects anyway and that there is a better way to deal with your problem. If you are happy to share your whole use case, the forum maybe able to help guide you better.
You just answered my this question
Phrased differently on StackOverflow; Not the answer I was looking for though but I do appreciate your effort and input. I added a comment to your answer which gives more detail as to what I am trying to do.