there are many other factors in speaker design that outweigh the inertia factor. I'm not saying (and I regretfully admit I made it sound this way) that inertia has no say in the LC equivalent of the system. Obviously it does- it's one of the main components. BUT, when you add in the system and the environmental factors, the inertia factor becomes far less important. All things being equal, a 15" sub should have a higher group delay than an ideal 10" sub.... BUT that's not the reason why generally 15's sound "sloppier" than 10s. The reason is actually just applciation. In general, who buys/uses/installs 15s? Folks who want SPL... those systems will be tuned for SPL duty alone and tend to have high group delay in order to achieve the peakiest (and loudest at a single frequency) transfer function.
