Is it not so much the specs, but the way the drivers deal with them? I seem to remember noticing that on PA speakers highs and mids are usually somehow 'favoured' over lows, even if the specs say that the delivered range starts at, for instance, 45 or 50 Hz. So if you use one for just bass, it doesn't sound as good as a 'proper' bass speaker, even when the cone is the same size.
That's why it makes a difference when a (sub)woofer is added to the PA system, in those cases where the bass doesn't have its own rig.
Looking forward to hearing the experts, of course.