I've never had the privilege of trying a 60s Bird of any sort, but I love the heck out of my '76.
The circuit on mine was rewired with the pickups in parallel like a modern bird, as opposed to the original series config. So technically, I've never tried an original 76-79 either... but this one sounds huge, open, with tight lows, bell-like mids, and highs with a warmth. It really cuts live, and its still one of my favorites to take out (although the Hamers and BCR are monopolizing my time lately).
Try out a recent one if you ever get a chance. You can find good deals on 'em, and they smoke. Nokturnal has a nice harem of these, and I'm still trying to devise a way to subtly abscond with his Blackbird
("is that a bass in your pants? or are you just happy to be here..."). Kidding aside, I've been tempted by the modern birds and BB several times.