You are on right track with the thickness, as thin panels only treat high frequencies. 6" panels would be even better. However, you can attain, (to a degree), a better sounding room by just using common furnishings such as pillows, throws and blankets. Also stuffed furniture. These things will quell and/or absorb some of these lower frequencies, improving your rooms response.
Question about acoustic panels
I'm looking to add some room treatment to tame what I would describe as muddy bass in a small room (12'x14'). I'm looking on the ATS site and they have acoustic panels in which they offer in the thickness of 4" at a price starting at $42.45. The description states "the added thickness increases absorption at low frequencies.
They also have 4" thick bass traps that start at $60.95.
For the savings of 18.5 per panel, could I get away with using the regular acoustic panels, or would I be better off spending the extra money and going for the bass traps.
Thank you.