How do I use my integrated amp as a preamplifer?

I have a Cambridge Audio 840a integrated and would like to use it as the preamplifer for Cambridge Audio 840w power amp.

The 840a does have preamp outs. However when I connect the preamp outs into the amp in, all I get is hum, and nothing else, even when I try to play music. I don't know what I could be doing wrong, or is this not possible?