I'm just asking if I miss some super secret sound absorbing forcefield generator that lets you blast in your flat and not aggravate the neighbors.
Well, it's not quite a forcefield generator, but there is a product for this, and Amazon sells it: http://www.amazon.com/Auralex-Subwoofer ... ex+subdude
It is an isolation platform which keeps the vibrations from carrying through the floor. Read reviews on it: normally, the story goes that the person installing it feels brave and turns the subwoofer up, and the next time he sees his neighbor a week later, the neighbor asks if he turned it down.
They make bigger (and uglier) platforms designed for guitar and bass cabs as well, which should work very well if your sub is quite big.
Apart from that, the only thing that will help is isolation. Cringe every time someone mentions "sound proofing", because there is no such thing. But there is isolation, and that is achieved primarily through mass, and secondarily through isolating vibrations between masses (think floating floors). Isolation and good bass is a difficult conundrum, since if you have super thick walls of very dense materials, like concrete, you reflect all of that bass back into the room, which creates serious peaks and nulls in the frequency response. You want
the bass pass through the walls, unless you have neighbors complaining.
If isolation is serious, construct two or three walls that don't connect to each other in a way that vibration could pass between them. The simplest definition is to "build a room within a room". But, if you're like me and live in a residential area, this isn't even remotely practical.
But for the love of donuts, don't cover your walls in egg-cartons. They diffuse sound over an extremely narrow frequency range and otherwise do nothing audibly. They are just an eye-sore and a fire hazard. (Real absorption and diffusion is another topic.)