Isolated ground outlets simply have the screw (green) for the ground wire separated from the metal body of the outlet, the ground is not floated (ie disconnected) its just separate. The bottom line is that in many houses the wiring is in metal conduit and uses metal outlet boxes, so when a standard outlet is installed there are two ground paths...the ground wire and through the metal box the outlet is screwed into. This can lead to noise since you now have a shared ground in all you house and this can lead to hum. If you have plastic outlet boxes it is my understanding that you already have isolated grounds.
Your situation is different though. Forget the isolated ground outlet, you don't have a ground wire. No Ground wire means floating ground and in general not advisable with equipment that is designed to be grounded (3 prong plug). You may have a house that grounds through the outlets, in which case the isolated ground outlet used without a ground wire is not the way to go.
Are all the outlets in your apartment 3 prong or 2 prong?
I would go to home depot and get an AC Outlet Circuit Tester plug ($5). You plug it into a socket and a set of lights tells you if the outlet is wired correctly and whether there is proper grounding.
PS audio makes a humbuster outlet to remove DC, which is probably your problem. Proper wiring is more important at this point though.
Cheers
