I have gotten far more mileage out of redesigning the power supplies, and rewiring the amplifiers in just about every case I've worked on.
Years ago, I had a Phase-Linear D-500 that was sensitive to buzz from light dimmers that use SCRs. One year, I tore the D-500 apart and rewired it. The manufacturer had not a clue about proper wiring practice. They had run all of the wires in one bundle, around the chassis. AC line, DC power, control wiring, and input audio wires--all in one bundle. I separated that, shortened all the leads and made direct runs. I even modified one of the PCBs, because there was a long trace that wasn't shielded on an audio input, and it was picking up hum from a nearby transformer. A strip of FR4 copper PCB, glued over that and connected to ground cured that. I also checked for audio across the power supply caps. Ended up putting 4.4 uF poly caps across the main electrolytics to shunt >1Kc signals that were backfeeding. Several things resulted: the buzzes and other strange noises from the line were gone. When the amp was driven to clipping, it no longer would produce a burst of ultrasonic oscillation as it comes out of clipping--the amp could clip cleanly for the first time in its life. It sounded better after those mods than it sounded when new.
My latest escapades into chasing noise came when I produced The Ultimate Fireworks Video on Blu-ray disc. It has an opening sequence with the national anthem being heard on a car stereo that was over 650' away on the other side of the airport, peaking at -85dB. Most Blu-ray players can't even separate this from the hiss and noise of their DACs. I had to get the Oppo BDP-83 to hear it without a ton of hiss. But every hum, ground loop, power supply rectifier switching pulse eddy currents, etc, were audible with such high gain playback conditions. I set to work on my Carver C4000 preamp, modifying the power supply, adjusting the ground return path to minimize switching pulses on the ground side, and installed snubber caps across the rectifiers. Then I went through the entire preamp and installed DC bypass caps on all the PCBs. I also put noise filtering in the power supply. The result is greatly reduced hum and noise. Now all the rest of the hum and noise come from the external components and ext processor chain.
The point of all this? It's not the power cord. It's poor design inside the equipment that it supplies power to. A good power supply design will filter out all the trash and noise that's coming down the line. And lord knows we have a lot of trash coming down our power lines, being in the middle of the aperture of a 2-bay ERI rototiller at 499' HAG driven by a Harris FM25K running full tilt for 50,000 watts ERP on 95.1MHz. When I built the house, I installed shielding in the walls and ceilings to keep the direct stuff out. The field is so strong that in our driveway, personal electronics that use microprocessors will stop responding to user input. That's right, a video camera will freeze up if you power it on in our yard, driveway, etc. We can hear rock music along with dialtone on our phones, too. And no doubt, it is in the power lines, too. So everything I modify has both RF filtering and AF filtering in the power supplies after I'm done. Hardly any audio manufacturer bothers with line filtering. Maybe an MOV transient protector, but no effort to screen out noise. It's really quite surprising. But worthwhile if you listen to high dynamic range content.