Today I got a Denon DRW-585 dual auto reverse cassette deck. I really love it, but before I try recording from an outside source, I want to confirm one thing: the owner's manual says that the record input can handle max 80 millivolts. Is this just warning against creating distortion or is it possible to damage the recording circuit by putting in more than 80 mV?

It is to limit distortion artifacts. There is no way any voltage below 5Vpp would do any damage even with very bad design (the Veb breakdown voltage of practically all transistors is above 5V, include those within ICs). Reasonable designs could handle 100'sV peak, given the time is short enough to not overheat the input series resistors. The active component then usually have diode clamps on their inputs. And there the 80mVcame from: To be far enough from a voltage, where these protection diodes start to conduct...

It is to limit distortion artifacts. There is no way any voltage below 5Vpp would do any damage even with very bad design (the Veb breakdown voltage of practically all transistors is above 5V, include those within ICs). Reasonable designs could handle 100'sV peak, given the time is short enough to not overheat the input series resistors. The active component then usually have diode clamps on their inputs. And there the 80mVcame from: To be far enough from a voltage, where these protection diodes start to conduct...

Thank you! I will do a test recording later.