When you say, "with other sources never a problem" what are you referring too? Other sources means cd players, dvd/blu ray players, TV cable boxes, all which would connect with the Yamaha v775. But when you say other sources, you seem to mean other avrs or receivers that drive your speakers.
At any rate, you may be experiencing something called ear fatigue. This is commonly experienced when the sound is not balanced or flat across the audio spectrum, but is overly pronounced in the higher frequencies. It's hard to know at this point what you are hearing though.
The Yamaha is a good brand and that shouldn't be the issue, yet your are experiencing something. Did you run the Yamaha room correction system?