I read an interview with Warren about this recording. Apparently they were very impressed with the bass sound on the record when they first started mixing, but when Allen would sing backup vocals the bass disappeared! They realized that the line in and the bass amp mic both failed and the only bass they recorded was through Allen's vocal mic!
It sure sounds amazing. I bought this disc the first time I saw the Mule back in 1997 and talked shop with him for a while. I have him to blame for my Gibson addictions...