If this weren't true the 67vdc to 80 volt rated drives on my machine would be cooking the motors, they only get get warm.
It's a combination of voltage and current that causes heat, and depends on the specific motors. Some run hotter than others.
How the motors are mounted also makes a difference. If mounted to aluminum plates, which is the recommended method, motors will run much cooler, as the plates are a heatsink.
At 40 volts I wouldn't change the voltage. Most run 48volts, and going to 36 volts would run a little cooler.
Hot steppers are normal, and if you can touch them without being burned, then they are probably not too hot.
Try the lower resistors and see if it helps.