Not from my own observations, but from quoted figures off the interweb. Mostly from the manufacturer. the 4" acoustimac does seem more effective below 400Hz than the 2". The 6" has a weird curve.
I have a room in the basement that I am planning on turning into a music room / studio. I will use it for playing the guitar and giving the rest of the house some peace. Recording, mixing and what I call mastering and others call ruining a decent mix.
Dimension are hardly ideal. 14'x14'x7½'...