Looking for a sub that you don't have to look at? The Bowers & Wilkins ISW-3 sub can be placed in a wall, floor, ceiling or even installed in a cabinet in your kitchen. The ISW-3 has a small (2" by 12"), slotted grille that looks like an HVAC vent, dual 6.5" Paper/Kevlar cone long-throw drivers in a balanced design eliminating transmission of vibrations through the enclosure and into the surrounding walls, and a -3dB point in the mid-20's. With the external amp, the setup will run you $2k. The question is, how much is it worth to you to have a sub that you can't see?
Discuss "Bowers & Wilkins ISW-3 In-wall/ceiling/floor Subwoofer Preview" here. Read the article.
As someone who has built & designed custom in-wall sub solutions for customers in the past, I can say there is a market for this kind of product. However in looking at the enclosure, it will not fit in MANY applications. The JL Audio & RBH solutions (amongst others) that are out are much more conducive to this style of application.
Although, I would rather build the sub for the application to ensure the highest level of integration within the space. AND, chose the compromises I want to deal with.