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Oops, I calculated an 8x8 .....oh well. My apologies for the bonehead math.
It hardly matters; the actual displacement differs so much from driver to driver and on the amount of surround (which is a considerable reduction in radius for 8" speakers) and on how the speakers react close to Xmax and the porting and... it's fun to do the math though isn't it? I mean, assuming the actual radiating surface loses 1" of radius for 10s but 1.5" radius for 15s, you actually have 12" and 6" speakers respectively for 226.08 vs 226.08" (OMG exactly the same I did not expect that) of total surface area, so the actual handling of Xmax and box loading makes most of the difference, as well as the amount of internal speaker volume lost to speaker magnets and hardware.
Whoa, they're the same.
I personally prefer (old school) 15s over (old school) multi-10 cabs because of the (usually) reduced high frequency response and correspondingly increased weird resonances around the cutoff frequency. In other words, the fun of old school 15" bass speakers is that they sound kind of honky and bad in a fun way (Mesa, certain Peavey drivers, etc). If they sound honky and bad in a bad way, I prefer 10s.
