If you want to know where he gets a LOT of the a cappella stuff, check out The Chestnut Grove Quartet, County CD 2709, 1994 (reissue). County also has a number of their old cassettes available as well, but I would start with the CD. Doyle grew up listening to the Chestnut Grove boys as his father was a fan, and was quite familiar with their work. You only need to listen to a few cuts to hear the debt his music owes to the Chestnut Grove Quartet. Check CDNow for song samples of their music. If the link does not work, just go to their site and enter Chestnut Grove in the search box. Tunes.com has song samples as well. I didn't look, but I bet you can get the album through Mudcat/Camsco as well!

Here's a quote from singers.com :

More people have heard about the Chestnut Grove Quartet than have actually heard them, for they influenced the world of a cappella music tremendously. The quartet was created by friends who had sung together in childhood in Washington County, Virginia where they learned shape note singing from itinerant instructors. After returning from WW II, they started singing together once again but just didn't feel the need for instruments and became one of those rare mountain groups who sang their hymnody a cappella. In the mid '50s they started singing on newly opened WBBI in Abington. From 2:00 to 2:30 each Sunday, for thirty years, they had a show, paid for by the sale of songbooks and LP's.(Their first album was recorded in less than an hour!!!) After the death of his brother and singing partner, Carter Stanley, Ralph moved back home and became a great fan of the quartet. He was so influenced that, in 1971, he began singing quartet style a cappella in his shows. It was a hit. Now most bluegrass groups put down their instruments for a moment of gospel harmonizing. This music is major part of the reason why that happens.