Thanks Balcro, that's fantastic.
So any of the drivers should work in that cab, just ideally in matching pairs. I think for the moment, assuming the tweeter was the cause of the nasty noise, I'm going to continue to use the cab with the current drivers, being careful to listen to any signs of struggling. I haven't actually turned it up very loud yet because if I turned the amp master past about 2 or 3, the sound which prompted me to post here occurred. I'm 99% sure that the sound I heard was not due to driver failure, but I'll test that at my next rehearsal. I think when I can afford to, I'll replace the OEM unit with a Celestion BL10-200x because that seems like the cheapest solution. I think the only way to find another OEM unit would be to buy another cab, which would make replacing the driver pointless unless I can find a cab for spares/repairs.
Thank you very much again for the response charts and checking to see whether the drivers will work in the cab.
Tom