I tried an exotic power cord and a power conditioner in two experimental conditions: clean-ish utility power, and power filtered through an isolation transformer.
Both products made a slight difference with utility power, but no difference whatsoever with the isolation transformer. Standard cords and the isolation transformer were the best, and cheapest, solution.
Plitron makes a very good one for a few hundred dollars - but it does growl, so you want to site it outside the listening room.