Have you tried contacting Denon for the manual?
As for the subwoofer, typically, home theater receivers are designed to give only a preamp level output for a subwoofer, as most home subwoofers have an amplifier built in. In the case of one that does not have an amplifier built in, you would need a separate amplifier to drive it. Consequently, the comparability (or lack thereof) of the receiver with 4 ohm speakers is irrelevant for the subwoofer, as that will be driven by a separate amplifier. The receiver drives the other speakers, not the subwoofer.