Subwoofer Bass Level Problems

What is your sub stood on? If it is on a piece of furniture then this could be the problem. Bass frequencies travel further than higher frequencies due to their longer wavelength and will also be transmitted through any solid objects the speaker is in contact with. The solution is to mount your sub on a piece of foam to provide some damping.

Also if your sub has a port (a hole) on the back or side this needs to have a good 3 or 4 inches clearance from nearby walls or panels.

Finally the speaker cone itself needs to have a clear path to push sound energy into the room, if this path is blocked by furniture (some people like to hide them under tables) then it may make that furniture vibrate.