Please explain. Given the fact that a watt is a watt, no matter what kind of amplifier, be it mosfet, digital, heck, even an SET tube amp (god forbid you see one in a car), a watt is still a watt. Now, given that, the SPL is purely generated by the speaker, which varies in efficiency. Ergo, I speaker of higher efficieny, at any given wattage, is the only way to generate more SPL from equal mounts of power, discluding impedance.
MOSFET amps typically run much cooler than a digital amp, and when an amp heats up, so do the wires inside, and that creates impedance, which kills power, which in turn hurts SPL. It would be more accurate in my eyes to say that the MOSFET amps are more efficient and typically will sound louder because they put out more power after being driven like most people will drive them.