adding more...
While a higher density form of rockwool is better at absorbing bass, you compressing it is not increasing its effective density, you are basically breaking it down to removing all effectiveness. The only way to get more dense rockwool (or fiberglass) effective for acoustics is to actually manufacture it differently.
Also, when it comes to bass traps, size is more important than "density". Higher density improves the bass cutoff for a single panel, but it doesn't make a HUGE difference at 40Hz. To make a difference at 40Hz you need to use 10x the surface area and 100x the volume of the material.