I would say that any coming from a '76 thru mid '77 should be treated as buyer beware, my two '76s are vastly different from each other in terms of tone. The pickups are the voice of a bass and mine do not speak with the same voice at all. As Scott has mentioned, Gibson was ramping up production in '76 and some basses were likely produced in Nashville before it shifted to Kalamazoo,MI., pickups were wildly different in output (mine are very much different!) by later '77 things seem to be a bit more consistent. Were I looking for a set of '76-'79 pickups the first question I would ask myself is: why is this set of pickups being sold in the first place?
I woodsheded with both of mine yesterday, turned the amp up and knocked the neck pickup back quite a bit and got some tight articulate tone out of them - certainly a different Thunderbird, I like that.
If your set up is this good, nothing else is needed, and I don't buy into the "increased sustain" hype either, none of us is Jimi Hendrix.