You can see impedance measurements of the D7200. It's nominally 20ohms, and peaks 5ohms higher than that, which is pretty flat. With a 10ohm amp there will be a 0.1dB variation across the frequency range, and with a 30ohm amp the variation will be 1.2dB - In other words, inaudible. Of course the amplifiers most likely don't all sound the same. Tomorrow I'll listen to the Denons and switch between 3 and 120 ohms on the same amp and report back.