Aredwood: The audio ground and the mains ground will be connected together inside the crack amp. And inside your PC. this means you have a loop that starts at the multi box or your houses earth wiring, runs to the case of your PC, through the ground wire in your audio cable to the crack amp. Out the crack amps power cable back to the multi box / house wiring. Using the laptop breaks the loop because Laptop power supplies even if they are 3 pin normally don't carry the ground connection through to the laptops chassis connection. 

Try the ground loop isolator from trademe to verify that it is a ground loop problem. If you don't want to use it permanently. The best way (I think) is to modify the crack amp. What you need to do is have separate audio ground and mains ground. Connect the metal panels, Transformer case ect to the earth pin On the crack amp. Insulate the ground of the RCA connections and the headphone connections from the case if not done already. Make sure that no other audio grounds are connected to the case. Get a high current bridge rectifier such as http://jaycar.co.nz/productView.asp?ID=ZR1324&w=bridge+rectifier&form=KEYWORD Connect the +and - terminals together. And the 2 AC terminals together. Then connect 1 side to your audio ground. And the other side to the mains ground. This will block any small circulating currents because the diodes in the bridge rectifier will only start conducting if there is more than 0.5V difference. But if mains power somehow livens the audio ground, The diodes which can handle 70A (2X35A in parallel) will easily conduct it to ground so the circuit breaker / fuse will still cut the power as normal.