David Tua certainly has a great chin, but he was still knocked down. He had the misfortune of boxing when Oliver McCall (one of the contenders for greatest chin, and IMO he is).
Someone once told me Tua, it may not look like the punches hurt that land on him, but he was definitely affected by them = look at the fres Oquendo fight. Fres was peppering him with soft punches but Tua couldn't get set to punch. It was Oquendo the busier and more accurate of the two, until he landed "that concussive punch in the later rounds".
I remember when Larry Merchant said there was “nothing manly” about Byrd’s fighting style when he was facing Ibeabuchi. But I give Byrd credit, he hurt Tua to the body in the 8th round when they fought. I was surprised by that. Byrd did a lot of that pitty-pat slap stuff which wasn’t fun to watch, but sometimes he would put more into his shots when he was going to the body.