If you have female thread on the speakers and on the Stillpoints, you need a step stud. You may find what you need at McMaster-Carr or Grainger.
Another possibility would be a threaded insert with 3/8-16 external and 1/4-20 internal threads. Then you could cut whatever length of 1/4-20 rod that you need.
If you can't find what you need, a machinist could make an appropriate length step stud from 3/8-16 threaded rod.
Another possibility would be a threaded insert with 3/8-16 external and 1/4-20 internal threads. Then you could cut whatever length of 1/4-20 rod that you need.
If you can't find what you need, a machinist could make an appropriate length step stud from 3/8-16 threaded rod.