Has the Sonos Amp been mentioned? It meets all the requirements listed in the OP. But physical volume control is not with a knob, it has up and down buttons. And you didn't say whether it needs to have controls to switch inputs on the device (you have to do that with the Sonos app, or it might have some autosensing that I haven't used). Analog line input measures poorly but I don't think it is too bad in reality.