It’s amperage that makes a machine run hot. It doesn’t hurt them to run hot but a power box should not be over 3 amps.
When I started tattooing power boxes did not have meters, digital display, memory or the ability to operate two machines. All these things are nice but completely unnecessary. What is necessary is your ear. When the machine sounds smooth it is smooth. To set the power don’t look at the numbers or even look at the machine. Close your eyes and listen as you turn the knob up and down on the power box. You will soon hear the sweet spot, forget the numbers, hear it. I find machines run best between 9 volts DC and 12 volts DC depending on the tension you put on the back spring. Personally I always put a bend on my back spring and use 12 volts DC to overcome the tension. My machines run very strong.
To test that your machine is running strongly, gently press your thumb against the armature bar. If it continues to buzz it’s plenty strong enough to drive a tiny pin into the skin.
In the early years of tattooing before hydro was common 12 volt D.C. dry cell batteries were used to power tattoo equipment. This allowed tattooists to operate at fairs and carnivals even if electricity wasn’t available.