Thank you for clarifying the same room same speaker position thing. The interesting thing is, if the 90 W (4 ohm) A19 could rock you Dali, how come the RX-A860 cannot? For stereo, that is, two channel driven, the Yamaha's output into 4 ohm should be comparable, may be a little less but not by much. There may be something wrong other than a power output related issue. You are also going by memory, not an apple to apple comparison so that could also be a factor.
Is your Dali Ikon 7 Mk1 or Mk2? I am not surprised they are not easy for the A860, but again you said the little A19 could rock them so maybe your room isn't that big or you were sitting pretty close. The A19 cannot deliver any more than 90 W into 4 ohms, less if two channel driven simultaneously. If "rock" means transparency, low distortion, nice overall sound quality then okay that's a different story.