I found this from an early-mid 20th century auhtor of the same last name:

Although Cabell's surname is often mispronounced "Ka-BELL", he himself pronounced it "CAB-ble". To remind an editor of the correct pronunciation, Cabell composed this rhyme: "Tell the rabble my name is Cabell."

I've heard John C. Breckinridge's middle name pronounced the same. Right, wrong or indifferent I say Cab-ble.