It is a mistake to say Russell was most influenced by Second Adventists (Advent Christians, Life and Advent Union, etc.) Russell self identifies as a millennialist Age-to-Come believer. By the time Russell met him, Storrs had left Adventism and was advocating a British Literalist theology. The same is true of Stetson. Stetson was an Age-to-Come believer, a Millenarian, and in his last years he wrote for The Restitution and The Rainbow, both Millenarian publications. Age-to-Come theology differs radically from Millerite Adventism. Russell also read and knew Thomas Wilson, Benjamin Wilson's nephew. Wilson's magazine is noted in the Herald of the Morning. Wilson was an Age to Come believer, not an Adventist.
While Russell believed Miller played a part in the divine plan, he rejected Miller's theology. He saw Miller as important only because he fit in a date system Russell inherited from Barbour. People often point to Barbour as an Adventist influence. However, by the time Russell met him, Barbour had switched to Age-to-Come belief and accepted Mark Allen's belief system. By the 1880s he associated his congregation, the Church of the Strangers, with Allen's Church of the Blessed Hope. Allen did not advocate any form of Millerism.
Much of what we might see as Russell's radical theology owes its development to British Separatist and Anglican writers. Henry Smith-Warleigh is an example. One can find elements of Russellite belief in various medieval era sects such as the Petro-Brucians and some of the Polish Brethren. You will find some similarities among some of the Paulicans. Some of his doctrine seems to trace to a second century sect called the Abrahamites.
It is an uninformed view that associates Russell with Adventism on the basis of his meeting J. Wendell. Russell felt Adventists were seriously out of the light of truth. You can find several discussions of the three principal approaches to Christ's return in Zion's Watch Tower. He says that Millenarians (British Literalist, Age to Come believers in the US) most nearly approach the truth. He was, as documented by Schulz and de Vienne, an age to come believer from 1870 to 1876, well known to readers of The Restitution. Schulz and de Vienne's next book considers this; their research is stellar.
You can trace Barbour's theology by reading Schulz and de Vienne's Nelson Barbour: The Millennium's forgotten prophet. Russell's Age-to-Come theology is traced on their private blog. You'd have to ask for an invitation. Expect to be quized and maybe turned down. They tend to pick people with professional credentials with a real interest in the subject. You can trace it yourself through the pages of the Age-to-Come preiodicals such as The Restitution.
Schulz and his partner have a public blog, though it is far less interesting than the private one. You can find it here: http://truthhistory.blogspot.com/
To read the private blog one must agree not to repost the material elsewhere. So, while I'd love to quote long sections of it, I cannot. I will say that it presents the most thorough, well documented research I've ever seen. I look forward to the publication of their next book.