I can appreciate that you don’t think that AC cords (or any cables) can make a difference in a system and are just shiny audio jewelry.
I am a big fan of older Yamaha amplifiers myself.
The models you listed are PA amps from the 80’s. Hardwired AC cabling and those (unfortunately) little speaker connection screws that Yamaha always seemed so hell-bent on using.
They have cool VU meters. In terms of raw power, vintage Yamaha’s are beasts. But they’re certainly not what I would call refined (or transparent) by any stretch. And unless you have kept up on maintenance (being that they are from the 80’s), I’d be willing to bet they are so far out of original spec that they’re probably not worth refurbishing back to manufacturer’s original values.
Which is the problem with vintage gear.
So, I’m not surprised that you can’t hear a difference. It’s impossible for you to try a different AC cable. Unless you’ve modified the amplifier’s speaker terminals, it’s pretty much impossible for you to try any after-market pre-terminated high end speaker cables as well.
I’m glad you enjoy your system, but your continual denial and arguing that cables don’t make a difference for anyone else in the world doesn’t hold a whole lot of weight.
I would be willing to bet that if you popped in a newer, higher-end amp/preamp, etc...you would not only be able to tell a difference, you would be floored by the differences.
But until then, you do you, but please stop telling the rest of us what we can and can’t hear.
FWIW, I'm a Jaguar kind of guy. 😎