You pose some interesting questions about the relationship between (rated) amplifier power and sound pressure levels from speakers. If you want/need to understand the basics about the relationships, try visiting this address: http://www.modrec.com/about/MRT_decibel.pdf
This is a .pdf file and you need Adobe Acrobat reader to view it (it's a free program) This simplifies the information somewhat, but you can see that in order to hear a noticible change in volume (3db) you have to double your amplifier output (i.e. from 5 watts to 10 watts) To increase the volume to what is generally percieved as twice as loud (10db), you have to increase your amplifier output by a facttor of 10 (i.e. from 5 watts to 50 watts). And to double the loudness again you would need 500 watts! Most of us can't afford 500 watt amps, so a good solution is to find more effecient speakers, which reduce the need for bigger amps. In normal listening environments (home use) - the real difference between a 70W amp and say a 100W amp translates to about 1.5db increase in speaker loudness. (all else being equal) What a bigger amp WILL give you is increased dynamic headroom and that WILL contribute to better/cleaner sound (unless you drive that amp into clipping) A speakers power input rating (i.e.200W) has nothing to do with its sound pressure output level. I too, would be curious to know how "loud" the speakers can play before they start to break up the sound - but I doubt that this can stated as matter-of -factly, since there are so many variables.
