Lots of bass at walls, lack of bass in center of room/listening position

Diffusers work based on wavelength. That's why you see those panels with wood blocks of different sizes at different heights. Each frequency has a wavelength. The higher the frequency the shorter the wavelength. Little blocks of wood can diffuse high treble because the wavelengths are on the order of inches. The low bass you're concerned with is at a wavelength on the order of some tens of feet. Ten to forty or more feet. In order to work your diffuser needs to be at least that big. This is why we have the low bass problem in the first place: even an entire room is small compared to low bass waves!

This is also why bass traps are no solution. They can do something, just not much. The only real solution is a room that is multiples of the lowest bass frequency. A room a hundred or more feet on a side. Think about the phenomenally good bass you have experienced at a rock concert. Because: big room! The next best solution is lots of subs. Each sub still has the same mode problem. But with so many each one puts out less bass, so its mode is small, they are in different places, and together they average out to exceptionally smooth powerful bass.