Since you're travelling in Business Class, you'll have access to the KLM Crown Lounges. Currently, showers are only availible at the Schengen lounge, which means you'll have to clear a security check before and afterwards. Not to worry however, as it doesn't usually take too long to change areas.
As 6 hours is a considerable layover, may I suggest the Mercure Schiphol Airport hotel, which is in the terminal. You can book a room for a select time period. Check it out below:

http://www.mercure.com/mercure/fiche...he_hotel.shtml

Good luck!