As mentioned put a post on the Denon Owners Thread where users of Denon x3800 can suggest speaker brands and models. Once you get this then ask the dealers for demo of those speakers only and no other models or brands. Once you are convinced then you make the purchase.
Else the other way is to audition speakers you like and make a list with their prices and specs and decide. If you need help in making that list shorter, our FMs will provide suggestion to steer you closer. Most importantly trust your ears!