An isolation transformer alone doesn’t necessarily filter noise. Some noise filtering can occur only if there’s an additional filtering circuit integrated with the transformer.
From experience, adding an isolation transformer before the main receptacle that powers the entire audio setup is one of the worst things you can do. All isolation transformers impart their own character to the sound — they slow it down and kill the liveliness. Maybe, just maybe, one could be used only for the turntable power supply, but I seriously doubt even that.
I know these are strong words, but in my opinion, there’s simply no way an isolation transformer powering the whole system can sound better than a direct, uninterrupted connection.