Does volts or amps increase the strength of an electromagnet?
Firstly, voltage plays no part in the strength of an electromagnet, it's only the current through the windings that generate the field. Consider a super-conducting magnet with zero resistance windings. There's no voltage, no power dissipation, and a large magnetic field.

However, most of us don't have the luxury of using a super-conductor to wind our magnets, we have to make do with a good-conductor, like copper. As it has resistance, we need to apply a voltage across it to push a current through it. This results in bad things like heating in the coil, whose temperature needs to be limited if it's to keep working.

For any given wound magnet, we do not have independent control over voltage and current. The magnet coil defines the ratio of those, it's called its resistance. If we apply a voltage, then (voltage/resistance) current flows. If we connect a current source, then the voltage across the coil responds, and becomes (resistance * current).

The limit for both voltage and current is given by the coil's cooling. If we operate the coil for a very short time, we can increase the power supply, and allow the heat to be stored as a temperature rise in the windings, as long as we switch off before the coil overheats, to let it cool down before the next pulse.