Question on NAD C 162 pre-amp

Just picked up a used NAD C 162 pre. (No manual) There are 2 sets of pre-outs on back of unit; one is fixed, the other variable with a small volume control next to it. What is the purpose of the two and which one should I be using? It's connected to a vintage Denon integrated with main-in. Also, when I adjust volume now it's a very smooth increase in sound levels, and I can go past halfway without "audible" distortion, and the speakers don't shut down. The Denon couldn't go past halfway without it sounding like it was screaming at you. Is this due to better control unit? thx