Questions most definitely welcomed and no I don't feel threatened if anyone questions what I am saying. Quite the opposite - I'm gratified that someone's paying attention enough to ask!
Firstly your question contains a huge 'if' which doesn't get borne out in practice. No two ground potentials are ever the same - at least never close enough for great audio performance. It might be true at DC but at AC frequencies which give us such grief (ultrasonic switching frequencies from PSUs) it can't ever be so due to inductance and skin effect. Well it could be so if there was zero AC current flowing - but that (drawing zero current) would mean we're not using the ground at all, so it might as well not be there.
I'm not familiar with ISO transformers so not at all knowledgeable about what benefits they might bring. Swapping the power supply in the computer is most definitely an option, but not for the faint-hearted. A linear power supply, preferably one using a split bobbin EI core transformer would be a big help - problem is its much bulkier and heavier Onot to mention runs hotter) than the unit it replaces, so might well need to be housed externally. But in reality the optimum solution is just to get some decent common-mode choking on the mains input - decent meaning ones with good rejection at the frequencies coming out of SMPSUs. For which purpose I've designed my own, not finding them in the current catalogs. More than happy to share details of the choke if anyone's a serious anorak enough to want to make one.