haradan, you're absolutely right.
We all remember Ohms law right?
Current (Ampere) multiplied by electrical tension (Volt) equals Power (Watt)
So when you undervolt a device, it simply draws more current.
Consequently, you can "undercurrent" a device, but then it needs more volts.
This is because the power usage in watts will be the same.
(This is not really how it works, because a device usually only draws the current it needs, notable exceptions are LED's and Fluorescent tubes needing a inductive ballast to limit the current i.e. a resistor.)
Everything is within limits of course, you can't run your MT-32 at 1000 volts of course
For instance my MT-32 has a ACB-220 power supply. This delivers 1200mA, or 1,2 Amps at 9 volts.
For the sake of argument lets say that the MT-32 actually needs all that power to function, that would mean it uses 9 x 1,2 = 10,8 watts.
So if you run it at 7 volts, it will need 10,8 / 7 = about 1,6 amps.
Consequently, if it gets 12 volts (which is not recommended) it will draw only 10,8 / 12 = 0,9 amps
Now because electrical components are rated to a certain maximum voltage you can't go overboard with this, but usually you have a 10% margin, and sometimes more.
But as you can imagine its safer to try to undervolt something than to overvolt something.
Here endeth the lesson.