Bass Driver Size - how much better is an extra inch?

Don't confuse frequency response with impulse response (impact) in regards to low frequency drivers. While a well designed 6.5" driver can cycle quite low in a well designed cabinet, it will only have a fraction of  the impulse response of a larger driver. Not all music styles require impulse response, like folk music or traditional jazz to name a couple, but impulse response is a major part of most modern music created since the early seventies and you will not get the full effect of that music out of a small speaker.

One way to get the transient response of a six or eight inch woofer out of a fifteen inch woofer is to put an appropriate fifteen in a well designed corner horn.  Admittedly this mandates a largish enclosure.  Mine are 18 cu.ft. each.

I have a 16' x 15' room with a pair of Klipsch K-horns in the corners of the long dimension (they're 1978 models that have an updated (per spec) crossover and modern wiring (lamp cord was original).
I've often wondered if I would significantly improve the sound by adding a subwoofer?

Emergingsoul wrote: 

"Have older b&w speakers and bass drivers are 7 inches vs 804 d3 speakers that are 6 1/2.  The larger drivers seem to really open the sound stage, more open and less constrained.  Imaging of older speakers not as good but a very enjoyable listen." 

Imo the difference between 6.5" diameter and 7" diameter drivers is inconsequential. 

I think you are SEEING a cone size difference and HEARING a sonic difference, assuming that the one is the cause of the other. 

My guess is that there are other factors involved which are not readily apparent, but which have a greater effect than the small difference in cone diameter. 
 
Duke 

I have older 803's with dual 6" woofers. My even older B&W's with 6.5 inch woofers do add something that is pleasurable. but I liked my old 8" Peerless subs that I put in my Acoustats better than the identically designed 10"ers I now use, because the tens are not quite as tight on acoustic bass. Interestingly, the difference between successive model preamps from the same company is nearly as great.