Nothing is wrong whatsoever with the Yamaha's amp putting out 3dB louder volume than the Adcom. The volume level is determined by the overall "gain" of the power amp sections of the Yamaha receiver versus the Adcom power amp -- in other words, what voltage of pre-out signal is required to get 1W output from the amp. This figure has nothing to do with the maximum power output available from an amp.

From your descriptions right above, I don't see anything is wrong with the Yamaha receiver.