Newbie Question

I recently purchased a Marantx SR5002 surround receiver. It puts out 90watts x7. I am happy with the unit. However, I have power hungry speakers with low sensitivity. My SR5002 has preouts. I was looking at the 75 and 125 watt Outlaw amps. However, Marantz technical support told me that if I used an amp with the receiver, I would have to hook up RCA [analog] cables to desired sources, and then to the receiver, in order to utilize the amp. They also told me that by doing this, I would lose all my digital surround modes. Could this be true? They said the reason for this is because my receiver does not down convert. I thought an external amp just took the place of the amp in the receiver, supplied the power, and everything else stayed the same. HELP!!!! I am confused.