You can never disable ground of the usb cable, it can and will eventually blow up the usb chip on the dac.
This can happen by accident when inserting a firewire cable and will blow up the firewire chip (this happened to me) it is a physical design flaw of the firewire plug were you can insert in a way ground does not connect first (before the other pins).
On properly designed equipment it should be no issue. Of all the devices I own(ed) only one gave a noise over usb problem, a digidesign mbox and that was a design fault. I thought my novation circuit had issues, but that was a faulty power supply.
So try first with all connected normally and worry if there is an issue and ask for help here.
Also, if there would be noise from the speakers, that is 99% of the time noise that reaches the speaker over the common ground and not from the dac which is designed properly, you will not hear the noise on the headphones out.

Thanks for your contribution.

Is it better to wire the isolator near the PC or near the DAC?
What are the technical parameters that a USB cable suitable for this use must respect?
Can you suggest serious brands that declare the characteristics of their models and don't cost a fortune?

Thanks again