Around 100 Hz, the most effective and easy to integrate would be membrane panels. Even those have to be quite large to work below 50 Hz. Larger than can be shipped. More likely a DIY project. Do a few weeks of research first and contact me if you have questions. There is a way to build them into the walls invisbly but it's fairly serious surgery.
I have built bass panels and the best I could describe it would be a maniacal "Mwahahaha".
Have no idea how a box within a box would work but it doesn't seem like a good idea.
I have built bass panels and the best I could describe it would be a maniacal "Mwahahaha".
Have no idea how a box within a box would work but it doesn't seem like a good idea.