If you are cranking up the power to 120 to 140 W, your receiver is running out of headroom. At 120W, you have less than 3dB of headroom. I can't find any photos of the capacitors to judge how much headroom the receiver may have but it's pretty crowded in that chassis and I am sure there isn't much capacitance.

Take a look at the capacitor bank in the ADA-1500 and you will see what I mean. Or one of the Bryston power amps.

http://bryston.com/images/products/28B3/L_4.jpg

Maybe you can retrofit one of these cap banks into your audio system smile

http://scott-eng.com/wp-content/uploads/2013/12/poleline-gallery-06.jpg