Romanticism followed on the heels of the Enlightenment to correct its left-brain lean. Its poetry of transcendent personal experience made good compost for SBNR. The sublime was directly accessible, there, all around.
(The Enlightenment itself was Romantic in its sweeping away of the old regime through revolution. And there had been previous SBNR milieus — particularly the Renaissance Academy of Ficino.)
Romanticism extolled experience over faith and produced important figures like Goethe and Blake who emphatically were SBNR. Looking East began — Schopenhauer, then Emerson. Myth was rediscovered, as was the spiritual ideal of romantic love.
Romanticism also encouraged excess — of emotion, of invention. Some initial fruits of SBNR springing from it, like the Theosophists and Rudolf Steiner, applied that excess to escaping the incipient mechanicalism of our age.
But there appeared also William James, and suchlike voices of reason, reining back the fantasies. And the 20th century’s Existential Occamism has reined in Romantic excess still more, very valuably. Direct heirs to Steiner like David Spangler now maintain a constant relationship with empirical accuracy. Excess does remain, but it’s of a different kind.