I'm able to get some 2'x4'x3" panels of Rockwool locally. I plan on making bass traps, (super dooper chunky style) out of it. The guy said it has an 8lb density. Would this be good for bass traps or would it be better to stick with the 2" thick Roxul Rockboard 60 or 80?

from this chart:
http://www.bobgolds.com/AbsorptionCoefficients.htm

The denser the rockwool the higher the NRC rating among Rockwool at the lowest frequency listed. Now whether that translates to a superior Bass trap I don't have a clue.

It also says that 80 is 8lbs density.

IIRC, somewhere on the acoustics master thread Terry Montlick comments that insulation can be too dense. Sorry, I don't remember the exact numbers, let search be your friend :)