I believe that the midrange is the critical range in a speaker system. By using a smaller mid woofer(3.5 to 5 inch) one gives oneself the flexibility to do lots of things and adequately cover the range from 150hz to 3500hz.Its important to know that when you are building you have the mid range adequately covered or your critical listening could suffer.
Someone on the PE forum when to the huge trouble of building a huge line array with their mid range covered by 6.5 inch woofers. As one might expect, the woofers really don't do the job adequately above 1600, and the their tweeters screech below 2500. So they have a very thin 1600 -2500 area.
Their only choice as i see it is to pull the baffle or add one, and pull the 6.5 inch woofers, and replace them with something smaller that can actually handle the midrange.
Marlboro