when it comes to speakers the room has the biggest affect on it's given sound. most good speakers are always measured and tested flat in an anechoic chamber. some speakers are tested in a diffused-field, but mostly nowadays tested on computers. there is more to speakers.
you got the cross-over,the cabinet, how well it's dampened inside,dispersion on and off axis of the tweeter,impedance vs. frequency, ect. many too variables to take in. it doesn't matter on the nominal impedance of dynamic driver(their actually electro-dynamic technically,especially the woofer) cause impedance varies with frequency. to determine the dips and spikes at certain frequencies is hard to say and best to ask the manufacturer of the speaker or if you bought drivers separate and building a speakers then best to ask the company that distribute that specific brand for a spec graph on the impedance. most manufacturers never display the speakers overall impedance. nominal impedance is only measured at 1khz. other problem is the cross-over. gets even more complicated there. the cross-over on how it's design will have it's own impedance handling as well.
power is not big thing with speakers as well. the whole wattage thing is thrown out of whack. it only takes 1w to reach the speakers given sensitivity@1 meter. rest of the power gets used up as heat in the voice coil and in the amplifier. it's called thermal runaway. when most people see 1000w or something they go ''ohhh wow'' and you have dummies go ''my speaker better cause it's 1000w''. most people barely listen above couple of watts depending on room size. most of it is all marketing cause unless the speaker itself(not the whole cab) is very,very heavy and expensive,it sure won't be taking a 1000w of input power. you'll melt the voice coil,possibility of killing your amp and starting a fire. power has nothing that determines the speakers overall performance.
distortion only happens if you feed too much power into the driver or if your amp is running out of current reserves and clipping. clipping is usually very rare unless your trying to power some PA speakers with little 10w op-amp based amp for a party lol.
you should only worry about speaker impedance when it's low(4ohms or less nominal) cause the amp needs to pull more amperes/current while trying to maintain a flat voltage output. keeping a consent voltage output is very important for an well design amp. if the amp is well built it should have no issues doubling it's given output power(like if it's 300w@8ohm, it should be 600w@4ohm, 1200w@2ohm,ect.)
