Using autotransformers

The term 'autotransformer' is a new one on me, and I have been a technician and engineer since 1974. I suspect that you mean something else. Which of the two below seem like what you are looking for?

An autoformer has no ability to regulate voltage. It can change the voltage, that is all.

If you want to adjust the line voltage, a variac can do that.

Al, we use such a device in our MA-3 amplifier http://www.atma-sphere.com/Products/#MA-3

Otherwise the amp would draw the line voltage down in a lot of situations and not make full power.

Depending on the power requirements you might be able to use an automated variac, but IME what is needed is an additional isolation style transformer that is much larger, and the automatic variac is used to buck the line voltage to it. This allows you to use a smaller variac and control a much larger amount of power that might be needed in a larger amplifier.

Ferro-resonant transformers don't work so well for audio. They make a lot of physical noise and electronic noise too.

Hard to say. Monster's website seems pretty bad, and does not say how much current the device can handle. You will have to call a dealer or such and find out.