i would say that everything that i've owned has benefitted from upgraded power cords but i agree that the effect on a powered woofer is not the most dramatic difference in the audio chain, though in my experience there certainly is an improvement.
the first time i put one on a sub (or a martin logan woofer) i thought that i was going to get deeper bass, but what i actually got was just a little more tuneful bass - less huffing, more notes. i unhooked my speakers, actually, and just listened to the subs (crossed over as high as possible) with different cords once, and when they had good cords on them, they were definitely less "wooly" sounding.
you have an easy way to check, though, just pull 2 audience cords off of your components, listen to a few very familiar tracks with the stock cords back in the chain to reset your ears a bit, and then insert the audience cords on your speakers and see if there is a discernable difference.
i wouldn't think that you need audience cords on your speakers, though -- in my system, for example, i can easily tell the difference between a stock cable and ANY upgraded cable on woofers, but it's much harder to tell the difference between, say, a $50 MAC or element cable and a $1000 shunyata.
give it a try, though, and let us know the results, i love this kind of thing. cheers!