maybe a bit late for input here.
when you separate the PSU from the amp keep the AC with the transformer and maintain DC inside the amp enclosure.
That means rectify and smooth inside the PSU. Then run DC lines to amp. More smoothing inside the amp and decoupling on the rails will help again.
I believe local smoothing and rail decoupling will affect the final sound quality of the amp.
Now my question.
All the decoupling (and local smoothing) should run on separate return lines to the master ground.
All signal returns should also run on separate lines to the master ground.
Where do you put the master ground? In the amp or in the PSU?
My preference is in the amp and run a PSU ground to the master ground (star earth).
The safety ground is only needed in the PSU and can be resistor connected to the PSU ground.