I generally look at TV related components long and hard. My experience, not all that extensive though, tells me TV stuff is a prime opportunity for a grounding issue to expose itself because they often use different grounds than the house electrical. I wonder if there might be a ground problem somehow being routed via the analog video from the Tivo to the Denon. I agree with Gonk that optical cables shouldn't (but it could!) be the problem; everything is plastic. You may have tried it but does disconnecting the component video cables between the Tivo and the Denon stop the hum? Otherwise I'd start at ground zero (everything is unplugged) and start adding components until you get the hum.