There are many different opinions on this topic; here's one of them:
Spend around 10% on cables for your whole system, for the Denon I would say around £50 for a good mains cable. If you purchase from a good dealer (try before you buy basis) or even a web based dealer that offers a money back guarantee (Russ Andrews for example) you can test the cable. If you feel it makes the £xx difference, then keep it. If not, get a refund.
I think that's the best way to go about it buddy..