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Ku hakemalla hain niin löyty tohonkin jotain kritiikkiä (http://mavenpeal.com/sag.html) :
 
The marketing term "power scaling" refers to an automatic internal variable transformer. While power scaling is impervious to varying wall voltages, it comes with all of the other problems associated with variable transformers.
 
Three problems with variable transformers include:
 
The more you turn down the voltage, the worse your amp will sound. This problem can be solved by a competent technician who can add a second power transformer to your amp to supply your heaters with enough voltage. The heater transformer is then plugged into the normal wall voltage, and the remainder of your amp is plugged into the variable transformer. While this approach is certainly Rube Goldberg, it does work.
 
The timbre and touch responsiveness of your amp changes as you change voltage settings. Specifically, your amp's power supply will sag less and less as you turn down the voltage. Your amp will feel increasingly stiff and bright, completely obliterating the reason for using the variable transformer in the first place--to enjoy 'cranked up to 10' tone at lower volumes.
 
You can't plug and play; instead you are constantly adjusting and readjusting. This is because you cannot set a variable transformer to a specific output voltage; you can only set it to a ratio of input to output voltage. When the wall voltage changes (which it does all the time), you need to sit down with your variable transformer and a volt meter to readjust the voltage to the output you want.