[Otto]

So the original setup doesn't work like it used to?

Initially, you said that the cones are moving a lot, but there's not a lot of sound. Could there be an air leak from the front to the back of the baffle? Something loose? I think you'd hear a lot of rattling if something were loose...

Can you do an REW sweep? What frequencies are moving the cones? Could there be some weird crossover problem?

If checking an amp problem, I'd definitely try to swap it out with another, known-good amp (even if it's not exactly the same model or power level). Perhaps put your IB amp on your mains and see if they sound weak. Does the amp have a bunch of configuration DIP switched like my EP2500? Could one of those switches be half-way in between a setting? Try swapping that amp around and see if it follows the amp...