Can you please tell me what optical cable you used? Struggled with this issue for years and if an optical cable somewhere in the chain can fix my problem i'd love to buy one.
there's basically only one kind of optical cable used in consumer audio (S/PDIF+TOSLINK, or however you want to refer to it). the trick is to arranging for both ends to use it.
GP probably has a sound card with optical output and speakers with optical input. This is usually called Toslink or SP/DIF (the latter may refer to a coax link - wired so won't help with ground loops).
The fiber is standard and you don't need any fancy Monster fiber costing $100 a foot.