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I am down in South Carolina. I think it would be a good change in your case. You may look at the SDA parts list to make sure the guts as all the same between the 1a and 1b. I would think the crossovers may be a little different, but it should be on the parts list. Listing the crossover question in the vintage speakers form may get you some good guidance if the 1a crossover issue if the parts list doesn't address it. All I have done so far is remove drivers, so everything else is still intact.
I have been trying to decide what to do about my sda1a's. The crossovers are rebuilt, and the speakers are great, but the cases and grills are seriously f,d up. As I understand it the tweeter arrangement in the B's is better anyway, so if the price is right I'd like to buy your cabinets and grills and transfer the contents of mine to yours. I'm in Bucks County Pennsylvania 18966. Where are you?