Well for one you can just make sure that all your ground wiring and anything that's connected to the ground is secured well. Then make sure that your ground is tied to the pin 1 of the Left and Right outs of the unit. I think the 'in' jacks should be tied to ground as well. You can also move the power transformer a bit to see if that helps. I hate to say it but there's a giant thread for the gssl at Groupdiy that will have you figure this out quickly.
On mine, I grounded every mounting point to the chassis and cut any loops formed by the ground traces. I then added considerable bulk decoupling. it got the hum down into the noise floor, but the unit is still one of the noisiest boxes I have overall.