The inside scoop on the SRM450's from a Mackie tech is that the power amp can indeed, produce 300 watts for the lows, but NOT at the current ohm load. I love the sound of the 450's, but they were far from perfect. The overheating was a constant threat in high output situations or when used on their sides.
Anyway - that's thw way it was told to me from a Mackie insider.
I doubt that they are optimized for the full 300 watts,..... just like the EV sxa100's - rated at $350 watts? Naw. These are made up peak numbers I think.