[MatrixDweller]

Resonance is when the sound waves bounce around inside a cavity. A resonant frequency is one that fits into that cavity and when bounced around starts to vibrate the materials causing an amplified effect. You do want the sound waves to bounce around because every hit off of a surface reduces it's energy slightly. That's what insulation does also, makes the sound bounce around inside itself removing it's energy. If you add a layer of insulation inside the cavity you would probably abate any resonant properties.

You have probably heard that providing an inch or two of air space behind a bass trap improves it's absorption. That's because the wave takes a bounce off the wall and goes back through the absorber. Sound waves also change when they go through different materials (wood, air, water, drywall, etc).

Rockboard is not horrendously expensive but it wouldn't be cheap if you were to stuff the entire ceiling cavity (10"?) with it you would end up paying more than $10 per running foot. In a 16x12 room that would amount to almost $1500 of Rockboard in the ceiling. You would be better to add a third layer of drywall or use mass loaded vinyl, both of which would be cheaper.