If you have an isolation transformer or a UPS, regenerator with floating secondary or output, not tied to earth, the most effective protection is a GFCI/RCD to the leakage current standard for your country, some are 30mA, some are less 6mA. Many are hard wired for panels, some are portable types. These devices are not expensive and are built in ratings from 6A to about 120A. As mentioned before, they trip on a difference in the current flow of both conductors being more than the tripping level of 30mA or 6mA. BTW, they are essentially a switch, with a core balance device that sits around the conductors, not through them, so for audio use, they don't compound sound quality issues.
Since both lines are above earth, both conductors must be broken when the GFCI trips, so use a 2 pole device, not a 1P+. Amplifiers, pre-amplifiers that have on off mains switches are usually double pole, due to dunces of this world mixing up live and neutrals on the input and getting things horribly wrong.
Equipment that has no integral on off switch has no issue. Any wall outlets that incorporate a switch must be double pole.
Any lamp that's connected to this system that has a switch, must also be double pole variety, and here is the problem.
It's very likely to plug in an appliance like a lamp into an isolated system, and switch only one pole, sure the lamp will go off, but the other side is still live. If there is a fault in the lamp, the GFCI will trip it. It's the inadvertent plugging in of an appliance that's not wired for an isolated system to cause regulation breaches.
A balanced system like the Equitech, BPT, Furman P-2400 IT, some Torus units need to have protection the same as the Isolation transformer above, that is have a rated GFCI on the output. I use a GFCI on the output of an Equitech Model Q, and it holds well at our regulation 30mA, no nuisance trips.
If your house only has GFCI in the bathroom, laundry consider doing the whole house. Many countries mandate the use of GFCI equipment for wall outlets and lighting circuits for protection of people and property in case of fires. If you go through a transformer, you need to maintain the same level of protection on the "other" side.