Lots of good info, but very little explanation! Let me try to fix that...
First, do you have mesurement gear? Can you measure your room's decay characteristics (waterfall or spectrogram plot)? This is critical to the selection of acoustic treatments, because all treatment options have a limited frequency range of absorption. The goal of room treatment is uniform decay rate with frequency. If you have a bass-only problem, and use something like the rafter treatment in post 12, you'll find it's sucked the life out of the mids, too. The treatment had broadband effect, but you only needed low bass.
Next, understand that acoustic treatments are not created equal. The design and placement of an absorber determines its effectiveness as a function of frequency. There are two general kinds...
- resistive absorbers, like fiberglass, that work by resisting the flow of air in a sound wave
- pressure-based absorbers that work by absorbing some of the wave energy, reducing the amplitude of the reflected wave
Now, let me digress into a bit of wave theory, so the next part makes sense. I think of a standing wave like a jump rope, oscillating between high and low in the middle of the room, but not moving much at the ends, where the walls are.
This is like the speed of air molecules in a sound wave; lots of movement, mid-room, but none at the walls.
The correlarry is that there are pressure maxima at the walls, and a pressure minimum in the middle of the room.
Resistive absorbers work best where air flows a lot, out in the middle of the room, away from boundaries. The closer you put them to a wall, the less low frequencies they absorb. However, this spatial dependence gives you the ability to "tune" the absorber response to your needs by simply spacing it away from the wall.
Pressure-based absorbers work best where there's no air flow, so at a pressure maxima like a wall or corner. These are 2-part designs, a resonant membrane or pipe plus a resistive absorber-filled cavity. Helmholtz resonators are identical to ported speaker enclosures, are tuned by volume and port size, and have similarly narrow band absorption. Diaphragmatic absorbers are membranes over resonant cavites, are tuned by both diaphragm resonance and cavity resonance, and generally have a couple octave range. Windows and deformable walls (sheetrock) are a common form of diaphragmatic absorber.
My room is naturally devoid of bass resonances because it's above grade, with wall paneling mounted on standoffs, an open attic above, and a 15' sliding glass door. No bass treatment necessary, but also very poor sound isolation; I can hear car doors, and I'm sure the sub is audible outside.
With a sound-isolated room, you minimize sound leakage, so you have to make up for the added energy that leakage would normally take away. If you're in a basement, you'll find that bass is reflected very well. The best place to start is still measurement, because you can reasonably expect to need absorption from DC on up, but have to account for thigngs like furnishings.
I hope this helps!