Tolerance information on Power Adapters going +/- 1 volt with same/different amperage

I need a post or link to a primer on tolerance information of going +/- 1 volt or more with same/different amperage and how dangerous that deviation is to 6V, 9V and 12V devices.

So I'd like to know the affects of using a power adapter that has
• Higher vs. lower voltage than the device asks for, +/- 1 volt vs. +/- 2 volts
• Higher vs. lower amperage specs than the device asks for

Background: An old Sega Genesis console says it needs a DC 9V 1.2A adaptor.
I had a bunch of 9V adaptors but they were 9V 200mA to 500mA range.
I also had a 10V 1.2A adaptor.

I resolved my original issue of SEGA Genesis console emitting a loud buzz, something it is apparently known to do, by trying several adaptors I had.
So once again the console says it needs a 9V 1.2A adaptor.

Buzzing went away with a 9V 500mA adaptor.
It was still there with a 10V 1.2A adaptor.
It was still there with a 9V 300mA adaptor.

What are the tolerances in general on this topic?

A cheap adapter has poor voltage regulation. I have one that is rated at 9V/200mA and it is correct. But at no current or a low current its voltage is higher than 18V. It is very hot at 200mA.
I have a locally made adapter that is 9V/500mA and it is correct. At no load or a low current its voltage is 12V. Its voltage regulation is not bad for having no regulation circuit.

I have a few adapters that say regulated 5V/2A and it is correct. From 0A to 2A the output is 5V.

The tolerance of the voltage regulation of an adapter depends on its price.