but if we want to calculate max SPL, amps are rated at a certain Wattage RMS at a certain Ohm resistance.

If we have a typical amp that puts out 100W at 4 ohms and 50 W at 8.
100W into 4 ohms is 5 amps and 20V.
50W into 8 ohms is 2.5 and 20V.

wow... well crap. the voltage is the same even at max output with different impedances, never thought about that. hmm... and at 20V the two parallel dirvers are each usign 50W... giving twice the efficiency..

don't I feel sheepish, I never really thought about how amps are limited.

I feel a little bit better that this doesn't defy any laws of physics, the speakers aren't anymore efficient in terms of pure power per output, but they make use of the amp circuitry more efficiently.

Thanks bud,
Danny