Assuming that both receivers were configured to provide 'reference' volume when set to 0dB, which is what most of those 'auto setup' routines will try to do, then there should be no difference at all between the two at 0, -5, -30 or -40. The difference would be that the old receiver would start to distort a bit sooner when peaks were reached.
Assuming that both receivers were configured to provide absolutely as much volume as they can produce when set to 0dB, then they should give exactly the same volume when the difference on the dials is 3dB. Not 10 or 15. Three.
Honestly, the difference is far smaller than the doubled power rating might seem to indicate.