I have a problem where,

My speakers hiss loudly when plugged into the wall socket in my entertainment room. I've tried plugging them into another room (using a long extension cord) and they do not hiss at all.

So the problem has been isolated to the wall socket.

I bought a $100 power line conditioner thinking this would alleviate the problem -- it didn't.

Is there any way to solve this problem _aside from_ using an extension cord to route the plug to another room.

Stumped here. Do you mean you plug your speakers into house current? I doubt that. Do you plug your amps into house current? I believe you do. So, my guess is the driving amps create the hiss as a result of drawing current from the outlet in that room. Seems it is not wired right. Get an electrician or someone who knows, not just says they know, how to debug bad house current. Could be not grounded, my first guess, or wired with the wrong cable, as in unshielded. This should not result in a Nobel prize as it seems straight forward.

You have identified the region where the problem is: that room. Check the wiring.

L8R