How hard is it to say the speaker and/or amplifier model...
Most likely you have groundloop on your system where the PC and the amplifier are connected to the same ground. This causes electricity to run in a loop between the PC, amplifier and ground. Optical obviously cuts the groundloop as there is no electricity or metal connections involved between the amplifier and PC.
There is also the possibility that your case was designed by monkeys
. But that should be rare nowadays.