Nashville, Tenn., — For a variety of reasons, it’s harder than you might think for groups of musicians who’ve assembled for tours and special performances to preserve their unique chemistry on tape for fans and historians. Studio time and expense, multiple commitments and sheer logistics usually overwhelm the noblest archival impulses. Even artists who keep every clipping, poster and video cassette from their career usually can’t produce audio documents that capture fully and professionally what these special and sometimes historic gatherings actually sound like, which is funny considering that’s why they came together in the first place….