A placed adventurer backpack has an insane blast resistance of 1200. It's even stronger than IC2 reinforced block.
Since it's a craftable block with high bkast resistance, players will use it to build base walls intead of reinforced stone or obsidian.
I would advise to use same blast resistance as a chest or, at most, a metal block.

I'd suggest making it as strong as obsidian, or as strong as that IC2 block, if you do make the change, to keep what the intent of keeping players' items safe when placed/worn.