Ok, so you are sure that the Power Item's guaranteed stat pass down isn't included in the 5 IV pass downs from Destiny Knot?
What?
Without the power item, all of the kids from your two perfect parents will inherit 5 of the 6 perfect IVs. The last one will be randomly generated and has the 1/32 chance of being perfect. For example, 1/6 of the time on all of your children, speed will be the stat that isn't inherited.
Let's say your power item is the one that passes speed. If you give this to one of your parents (while the other is still holding the destiny knot), then the speed stat will always be passed down. However, like I said before, only 1/6 of those children won't inherit the speed from the parents (because of destiny knot). Those children will now always inherit the speed
in addition to the other 5. So you'll end up with a 1/5 chance to get a 6IV child.
But like I said, you won't be able to control the nature. With 25 possible natures and the 1/5 chance of a 6IV pokemon, with destiny knot + power item, you'll have a 1/125 chance of breeding a 6IV pokemon with the nature you want. With destiny knot and everstone, the chance is only 1/32.
Make sense?