As a sound engineer, I'm curious what a "voltage/capcitance/current/frequency issue" is? I was mostly with you up until the frequency part.
The things that have been added to stereos (mostly surround processing, some simple source switching, and D/A converters) since the 70s aren't huge power sucks to the extent they would cause amplifiers to sound worse.
it's like saying "Lets add a 1000W lamp to this wall socket and not expect anything bad happen to the Audio on the same circuit."
As long as there's not a dimmer involved leaking into the circuit, a 1000 watt incandescent light sharing a circuit with a home stereo should have no effect unless you have a ridiculously loud and/or inefficient stereo.
Talk to any sound engineer (read non-audiphile subscriber) and they will have tons of stories on how fickle sound set ups can be when no one knowledgeable is watching the setup and correcting things.
Watching the setup and correcting things? You set up a system and pretty much just use it. You might re-tune a PA system once there's bodies in the seats acting as diffusers or raise or lower the overall volume based on the size of the crowd. But it's not like you sit there adjusting things about the amplification system itself as a matter of course during an event.
The big factor is that people just don't care about audio sounding good so manufacturers have cheaped out to save themselves money.