My rule of thumb is "if you don't know why you need to upgrade, then you probably don't need to upgrade." Add external amplification if you know what it improves - for example your speakers aren't getting loud enough and you've heard these same speakers at a friend's system using external...
Maybe this very positive assessments of Denon AVRs (yes, better than Marantz) can add some more context to your buying decision. I interviewed Amir of Audio Science Review yesterday, and specifically asked him about why he holds Denon in such high regard: