To claify what Mick said: they don't exactly stack, BUT if someone uses a bludgeoning weapon to bypass your DR 15/bludgeoning, they would THEN be subject to the Stalwart Defender's DR 6/ (not having a descriptor after the slash means nothing can bypass it).
Ah, that makes really good sense. That would actually be pretty beneficial. Hmm..
Another question: Why does Weapon of Impact not work on creature weapons? I notice a lot of other spells were changed to work with them, but not this one?
Does Whirlwind Attack suffer from attack bonus loss if improved combat expertise is activated?
The description says it "makes one melee attack at full base attack bonus" so hopefully that means they make a good combination when playing defensively.
It is still a good defensive option, of course - as long as your full AB -3 or -6 is going to hit the enemies you are fighting. You also get regeneration from vampiric weapons, unless I am completely mistaken.