wattage ratings on speakers arent really about the loudness.. they are referance to the amount of power that can pass through the driver without melting the components breaking the driver down and causing it to fail. this has no bearing on the sound quality or distortion of the given driver at that watage.
my general rule is avoid clipping. unless your driving your amp hard then you generally wont run in to clipping issues. clipping is a killer of speakers more so than running the speakers with a higer wattage amp, this is mostly due to the fact that the amp wont always be running its full wattage through the speaker constantly due to frequency variations.
if you run your amp at full volume for a lot of the time because thats the volume you desire then you probably need more wattage so you can drive your speakers at a lower setting creating the volume you desire without the amp clipping some of the sound waves creating flats instead of curved waves. (speaker sizes and db ratings may also be a factor to consider 1st as a better driver will need less power to move the same amount of air.)
if you dont have your amp at full power most of the time and the volume is good then you do not need more power as the amp drives your speakers fine for you.
the whole thing about speakers reproducing sound is they need to move backwards and forwards to create the air pressure waves, your amplifier sends power to the drivers which moves the driver 1 way then the other way. when the amp moves the cone forward the more the cone moves the harder it is to stop it to make it move in the opposit direction. in an ideal world there would be zero effort to change the cones direction.
the main thing "other than clipping" that i worry about is impedance, its better to have 8ohm than 4ohm (imo) as most transistor based amps will have a lower THD at 8ohms than they do at 4ohms, and even though you could be looking at 30% more power running at 4ohms, at higer volumes this is virtualy no diferent that 8ohms, but it will distort easier. and enter clipping easier as the amp has to work harder.
this is not generally a limitation of the speakers them selfs, many 4ohm speakers will soundjust as good as 8ohm speakers these days, but amps tend to like more impedance to avoid clipping.
so basically thats my answer.
You dont want the cone to move much its inefficient.
if you have your system volume on really loud all the time entering in to the realms of clipping to get the desired db's out of your system then you may want to look in to drivers with a higher db output. And only after that think about more power.
when it comes to the actual sound you want to hear i would always reccomend 8ohms over 4ohms.
if you really must see drivers moving (even knowing they are less efficient, harder to drive, and could well distort and make a horrible blowing noise) then i would have to agree with others that a sub woofer is the way to go.
as for the question of how much power do you need to power a speaker?
really depends what it is and what your amp is like,
I dont think there are many drivers you could not power comfortably with 50w Rms but again if you arent providing enough power for your audio pleasure you will crank up the volume which could enter the realms of clipping.
This is just my un educated version of how these things work. if i need to be corrected then i would have no issue with that
