Regarding the output transformer; it’s important to keep it toward the “power supply” end of the chassis, away from the input jack and to position it so the coil winding are perpendicular to the power transformer as much as possible. There are many explanations about transformer placement on the web, following the recommendations, you will minimize the potential for the transformer placement creating or contributing too hum and noise. It is possible to “tune” transformer placement for the lowest noise by moving the transformer around (slightly) on the finished chassis.