Yes speakers do sound different due to their design parameters, and some 10"s can go lower than some 15"s - its about design. At its basic basics, a speaker with a heavier cone will have lower resonance and handle lower notes than a lighter cone - but you'll lose volume (efficiency) Don't now have a go guys - I know there's a lot more, but that's the base basics.
So you could build a cab with all 10"s or a pair of 15"s - its the speaker design that will determine which goes lower or/ and louder. But the cab also has an effect and can work with the speaker or against it to 'frame' the function of the chassis. Most of what goes on in the commercial World is about compromise to get a cab that doesn't become a pain for the distributors to handle - ie too big, too heavy, too expensive, too difficult to build. This approach gives us a wash of smallish boxes with or without ports. When was the last time you saw a bass scoop cab on sale?
The contra market approach is like BFs cabs with recommended chassis's that are way more efficient (like twice as loud for the same wattage) but generally bigger dimensions, more complicated to build, and would there fore be way too expensive to seem competitive to the general marketplace. And don't forget that 95% of the market is beginners, intermediates, parent buyers, and schools - they all buy at price points.
Bottom line is, if you are a serious / pro player you'll probably go for the expensive end and / or get something built for you like Bill F's cabs with high end drivers like the 3015s.
