I was debating this with a tech at Protech (repair shop in Silver Spring, MD, where I used to be a bench tech and now work part-time doing ebay listings) and neither of us could find a logical answer. If you have two speakers, identical except for impedance - one is 8 ohms the other 4 ohms - and you plug them, one at a time, into the output of a solid state amp (tube amps have output transformers so I don't think this'd apply) would one be louder than the other? Theoretically, a 4 ohm load would allow an SS power amp to put out about twice the power. Would the 4 ohm speaker then be louder? I was thinking that a 4 ohm voice coil would have fewer windings and therefore need more current to move it in the magnetic field, which would make the SPL about the same for either driver.