Without pre-amp outputs from the Cambridge, you'd need to use the "rec-out" and find a power amp with volume controls (fairly rare).
You're other options would be to add volume controls (passive pre-amp, attenuators, or an active pre-amp with another set of interconnects) between the Cambridge and the power amp.
Either way, you're only using the Cambridge as a multiple input (source) selector, so it seems pretty pointless to me.