Be sure not to neglect the late, great John Fahey. Labelled as an 'American primitive', the man was certainly a visionary and straddled numerous genres with much experimentation & extrapolation. His earlier works focus on acoustically based recordings and are nearly all worthy of your attention. I would also heartily recommend "Of Rivers and Religion', a mid-'70's recording with a full band that dove into a strange nether-region of languid guitar strum/jug band fusion that to this day seems timeless. Sandy Bull (also recently passed on) is another great one. His first two albums, "Fantasias for Guitar & Banjo" and "Inventions" are great and, although featuring some electric guitar work are primarily acoustic guitar-based and offer up an amazing blend of classical, traditional and avante-garde stylings with the heavy influence of a variety of ethnic musical stylings. He even features an oud on many recordings and admits a serious debt to Hamza El Din for much of his inspiration for some of his classic pieces. He would be legendary just for his 'Blend' (from "Fantasias") and 'Blend 2' (from "Inventions") compositions if nothing else. Robbie Basho, a compadre of John Fahey who also recorded on his Takoma record label did some fascinating work along a similar path. Heavily influenced by eastern philosophies and musical idioms with a complex and very lyrical feel. The above musicians veer away from the path of the more classically based composers offered up in this thread, but are well worth looking into.
