We have seen JDS' chin tested. He has the best chin at heavyweight, period. He took a huge hook from Yvel, shots from Crocop, and shots from Nelson and it didn't even budge him. The hook he took from Yvel would have finished most people at heavyweight IMO and it didn't even flinch him.
Plus he's never been dropped or rocked in 16 kickboxing matches and all his pro fights.
sig credit to Limba