Arianism was prevalent among the Adventists in the mid-19th century, even among the leaders. Arianism entered the Adventists through the Christian Connection, which later merged with other groups to become the United Church of Christ. There is a offshoot of the SDA that is Arian, and they have a website about how the early SDA leaders were Arian. Perhaps I can find it. The Adventist group Church of God of the Abrahamic Faith are Unitarians, but do not believe in the pre-existence of Jesus. They were influenced by the Christadelphians.
Storrs believed that Jesus was God, but believed that the relationship of the Father and the Son was a mystery, and would not characterize his belief as either Arian or Trinitarian. Russell was an Arian in his beliefs like many Adventists of his time. Russell likely got his Arian views from George Stetson. Stetson was an Advent Christian minister who was pastor of the Allegheny Advent Christian Church from 1871-1873. Stetson wrote an article on the Ransom that appeared at the time in the the Advent Christian magazine. He gives the view that the ransom was a corresponding price to Adam. It is on the internet and I can locate if anyone is interested.
The SDA, Adventist Christian Church, and the Worldwide Church of God all made moves toward accepting the Trinity, reversing an earlier anti-trinitarianism.
Steve