Isolar made a good observation. The preouts on the new Yamaha are downstrem from the volume control, at least I'm pretty sure they are, so unless you have the volume knob on it cranked you are sending a much lower voltage signal to the Adcom than you were previously (sending a full hot line out from the N303 and relying on those Niles thingamadoodles to trim the volume back on the amplified, distributed signal, way downstream...we really need to talk about that whole part of the rig. It sounds like an install from the 1990's.)
Max out those Niles volume controls, full wide open, as you're now controlling the volume from the Yamaha.