You're obviously a fEARful fan, but their 2x15's that I've seen have two 6" drivers as well, as to not lose the mid articulation![]()
There is also the Dually with no 6" drivers and without a tweeter or with one.
Obviously no two cabs are the same, but generally speaking you're going to get more volume out of a 2x12 than a single 15", and more volume out of 4x10 than a 2x12; it's a simple matter of speaker surface area.
It is a matter of speaker surface area AND excursion, the product of which is displacement and it really isn't that simple. Speaker excursion values vary so widely that you can not speak generally about the volume from different cabinets. For example, I have compiled a list of specifications of various cabinets from different companies and the sensitivity values for 4x10 cabinets range 98 to 103dB. For 1x15 cabinets it ranges from 96 to 99.5dB. Those are rather wide ranges I think. Far too wide to generalize about.