the amplifier ground still goes through the power supply board. the amplifier ground should also go direct to the star ground as well to eliminate any voltage drops between the "left side" of the power supply board and the "right side" connection where you currently have the amp ground connected.
Now have the ground wiring in the amplifier connected as in the photo, and that immediately yields a better image on the oscilloscope, without input and with 8 ohm load resistor.
Will quickly take some pictures of the square waves and now look clearly better.
The Kanede amplifier is now working and sounding well, here a number of photos of the square wave display amplifier loaded with an 8 ohm resistor.
Photo 1-40 Hz.
Photo 2 - 100 Hz.
Photo 3 - 1 Khz.
Photo 4 - 10 Khz.
Photo 5 - 20 Khz.
Photo 6 - 50 Khz.
Photo 7 - 100 Khz.