I have held off from this debate because, as a Brit, my knowledge of SBC history is limited.
However, may I ask if James Petigru Boyce and John A. Broadus are accounted as founders or early members of SBC?
On another topic, to be Reformed is to be Calvinistic, but to be Calvinistic is not necessarily to be Reformed. If one is 'Reformed' in one's theology, one believes not only in Definite Redemption, but also in Covenant Theology and in some form of the Reformed Principle of Worship. I believe the term 'Reformed Baptist,' as it is understood today, is as recent as the 1960s. Walter Chantry and Geoff Thomas coined the term when they were Baptists studying at Westminster Seminary.
The SBC came out of the Triennial Convention. This was the founding (the SBC was formed as it split from Northern states). Calvinism was not a distinctive. The distinctive was missions (an opposition to Primitive Baptists, particularly the anti-missions movement). The Triennial Convention stated it would take no position on slaver but this proved impossible and the SBC split forming the Northern Baptist Mission Society and the Southern Baptist Convention.
There were a range of beliefs when it comes to Calvinism. But the SBC was not founded on any of those views.
The first leader (first president) of the SBC was William Johnson. Here is what he said on the topic:
"As a Free Agent, man has life and death set before him, with the liberty of choosing the one, and rejecting the other ... As moral agents, for whom there is hope, I call upon you, then, fellow sinners to turn to the Lord ... Now, Now, O fellow sinners, you have it in your power to place yourselves under influences, that are spiritual and saving; or under influences that are carnal and damning."
Provided the R.B.C. Howell’s (the 2nd president of the SBC) comments that the SBC departs from what he considers “Calvinism” (although I believe him a moderate Calvinist in practice….he struggled against the anti-mission movement which may have colored his language).
And we know that Brantley (a SBC founder and pastor of First Baptist in Augusta Georgia where the SBC was formed) was very much opposed to the idea of Irresistible Grace.
My point is not that the SBC was “non-Calvinistic”, but it is wrong to say it was Calvinistic as well. The SBC does not prescribe such doctrines and have always been diverse when this topic comes up. Some wish that the SBC was founded on strong Calvinism (it was not) and others wish it was founded on strong free-will theology (it was not). It was (and is) comprised of churches that hold a wide view on these topics.
And thanks for your view of how "Reformed" differs from "Calvinism".