In the end I just used the '76 Thunderbird only. The only reason is that I'm so used to that bass. It just feels good.
But I did a good A/B test of both Birds and again, the BaCH sounds awful goog!! Very much like the '76er but with a bit fatter bottom. The '756 is a MidMonster and the BaCH has some warmer lows. But definately family. Both sound great on a high volume Hiwatt.
Maybe I use the BaCH for our vid.