Low frequency sounds are non-directional to humans. That’s why you can get away with putting a subwoofer under your desk and not have it wreck your stereo imaging.
Subwoofers reproduce less of thr frequency spectrum than you may be thinking. Try turning off your small speakers and listen only to the sub. You won’t hear any high frequencies. My sub has a cutoff filter that can be set from 80hz to 150hz. Maybe some cheaper subs are more like full range speakers, but most aren’t because once you get above 250hz or so you start to get directionality, and hear that the lower notes are coming from somewhere else.
I drive them with an amp I bought for $50 from a pawn shop. They are driven from the subwoofer output.
I won’t go without these things again. Not only do they really shake the couch, but they give an illusion of loudness. Turn off the shakers, and everything feels quieter. But the cool thing is that you obviously don’t hear them when you aren’t sitting, so it’s kind of the same as having better sound isolation in the room. In particular you don’t need as much bass, and it’s bass that typically travels and annoys people in the house.
Best upgrade I bought for bang for the buck. The total was like $350, plus I had to cut open the bottom of the couch and install them and run wiring. But they are awesome. A single one under a gaming chair is also great. And action movies are awesome.