also, after looking at the diagram for your ground connections, it looks like you have star connections between the capacitors, which is good, but some of the other ground connections are branched, and the amp ground goes back through another board with a ground. if there's current between the two ground points on the power supply, and the amplifier ground is on the far side of that voltage drop (when you have current flowing through a wire or PCB trace, the very small resistance develops a voltage across it, and one end is no longer truly at ground potential), the ground reference for your input stage has changed, and that can cause oscillation. the amplifier ground should go directly to the "star" ground between the power supply caps. there should also be two separate speaker grounds going back to the star ground. there are books about amplifier design that cover the whole subject of ground systems, what works, and what doesn't work.
ground layouts can be tricky to get right, and probably has a lot to do with the opinion that many engineers have that "audio design is a black art". even in the 21st century, receivers and amplifiers go into production, and within a month of the equipment hitting the market, manufacturers come out with a flurry of ECO's (engineering change orders) and modifications of "cut this PCB trace and run a heavy gauge piece of wire from point "A" to point "B". "