The wavelength a panel can absorb is directly related to its thickness. Location also makes a huge difference in terms of absorbing mid and low frequency energy. You probably want to add some big thick bass traps to the power corners of the room (ceiling/wall corners and floor/wall corners). I am happy with the panels I got from ATS Acoustics - you can also have them print custom images/pictures onto the front of them so they look like wall art vs. acoustic panels.