from year to year Yamaha would make P/S changes in certain models which would cause problems even though the previous version worked with out any problems.
Thats exactly what Tom at Axiom told me and he seems to know some if the engineers a Yamaha.
Also, if you look closely at the specs for the 559, 661 and 663 you will see that they have different max power outputs at 8 6 and 4 ohms.
The 559 was shown by audioholics to be a
very beefy receiver, the 661 not quite so powerful. On paper, the 663 looks closer to the 559, but until someone does some testing we won't know for sure.
Whatever you think of the power side, they sure hit a home run on the features.
