Lots of acousticians on Gearspace argue that the quarter wavelength rule is misleading as it's the depth of porous material required for 100% absorption of that frequency. They argue that thinner depths (e.g. 40cm) will still provide meaningful levels of absorption at frequencies down to around 50 or 40Hz as long as enough surface area is covered. It also gets much easier if you're able to position the porous absorption in a high velocity area for the offending frequencies (i.e. stood off of the wall), which is where they work best. I managed to shave 1dB off a 43Hz axial mode peak and 7dB off a 75Hz SBIR null with a basic 17cm thick porous broadband absorption panel stood 2ft off the wall, - an impractical location for most people. A more efficient method of absorption requiring much less surface area coverage would indeed be diaphragmatic, limp mass membrane or Helmholtz, which can be more conveniently located at the room's boundaries.
Attempting to treat an amplitude LF peak with absorption is an expensive and space-consuming endeavour, and you can get the same if not better results by moving the listening position and/or applying a simple parametric EQ notch filter. This, however, will not ameliorate modal ringing, nor will it give you a smoother response throughout the room with less amplitude variation between peaks and nulls, so it depends what the desired outcome is.
