For the most part, watts are watts, and any given amp of the same technology (class D, for example) will generate approx. the same heat output for the same power level.This amount of heat is based on the efficiency of the amp. When I say same, I don't mean actual temperature of the heatsink, but rather actual BTU's, or deg. per sq. cm, output.

Now, an amp designed to push 2000 watts at 4 ohms will probably have more heatsink than an amp designed to push 2000 watts at 1 ohm. So, it will run at an overall cooler temperature.

In your example, I would expect that the 1 ohm stable amp that puts out 750 at 4 ohms will be a physically larger amp, and would therefore run cooler in overall temperature.

This is probably over simplified, but I believe it is a good rule of thumb.

Toby