Maybe what was meant is;
He is using a plugpak/wallwart so there is no mains to the amp?
"IF" so and there is hum/buzz then the Q is valid but confusing.

@ Flester;
The term *Ground* can have many meanings.
Mains ground (EARTH) although related is quite different to the Ground or *Common* in your circuit.
If your Amplifier is as above, no mains in the chassis and powered via an external double insulated supply then you have have a local circuit ground issue.
Grounding back to mains EARTH probably won't solve much if you have a circuit problem causing hum/noise.
If it's hum, You most likely have a bad connection in the amplifier circuit common path somewhere.
If the circuit is very buzzy then likely there is too much gain somewhere with bad shielding.
Be aware that a lot of amps for modern higain thrash will all buzz when the distortion pots are maxed out and at bedroom volume it quite pronounced.
At gig level you would hardly notice it until you stop playing.

And yes,,, as *DrGonz* mentioned give us more info and clarify the problem so we can help.

Phil.