Low level 50 Hz Buzz in my G SSL Clone

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.