While looking for ways to get to the Doug Seroff collection that Polymando mentions in the "New collection at Center for Popular Music at MTSU" thread, I somehow ended up here:
http://blessmybones.blogspot.com/,
which has "African-American gospel music from the turn of the century to circa 1960, with an emphasis on the great quartet tradition."
Lots of great stuff to go through, many audio files of quartets, famous and obscure. Take a look.
Lindy