You should apply your voltage
supply to the amplifier through
connector. This connector is a LPF that filters
out any noises that are present in the supply rails.
The connector is screwed to the case, and should
be located close to the output of the amplifier
in order not to add noise to the sensitive input.
The shielding of the wires should be connected
to the aluminium case. You should form a perfect
(As much as possible) ground plane from the input
of the amplifier to its output. The GND of the supply
voltage should be in the same potential to this
ground plane, therefore you should connect it too
to this ground plane. It is a good idea to make
different compartments for the regulator, amplifier
and transformer and by that to decrease potential
coupling between these elements.