I really doubt if a $1000 device comes with a power cord that isn't able to handle a 670 W device. That's less W than the average vacuum cleaner coming a a power cord worth a few dollar.
And I can't see what can be upgraded to power cord.There is no need for it to be better (what would be 'better') than the power lines hidden in the wall of your house. And those certainly aren't USD 400 for 1 or 2 yards.
I purchased a Denon x4000 from a local home audio store and they are suggesting I buy a upgraded power cord for 200,300 or 400 dollars. They say it will make a big difference with the sound quality. True or false? Any input would be appreciated.