Just because someone puts in a digital isolator doesn’t mean they have isolated an input. Even if they also provide isolated power to the USB processing doesn’t mean the have isolated an input. Digital isolators come in many forms and most actually have problems of their own (e.g. high rate clocks internally, too much capacitance allowing high frequency noise to pass right thru or adding gobs of jitter.) Remember digital isolators aren’t normally built to keep noise out, they are designed to keep large voltages out, something entirely different. In the DS and many DACs there’s precious little jitter or noise in the data after the USB is processed, but merely having a USB signal coming into the box radiates noise thruout the DAC - i.e. it’s not necessarily an isolation problem that isolation chips are designed to help. Not that I’m saying doing good isolation is a bad thing, but (like most things in audio) it’s not a panacea.