I still can't get the concept out of my head. I want to tackle a DIY SonoSub, but I have restrictions: Hers: It must be invisible. Well, OK. Less visible. The smaller the better. Mine: It must be accurate and reach lower than what I have. Ideally, down to 20Hz or lower. These points keep leading me back to an isobaric sub configuration. Please correct me if any of these observations are blatantly wrong: 1) Ported generally means larger. 2) Sealed generally means smaller. 3) Sealed enclosures don't reach as low as ported enclosures. 4) Sealed enclosures have a flatter response than ported enclosures. 5) Isobaric means half the volume of sealed. (Actually, I'm surprised to see how many people have built ported isobaric subs - when I thought they were supposed to be sealed enclosures.) 6) Isobaric configurations are less efficient. 3dB? 6dB? Doesn't seem like much to me. Power's not a problem. I have a collection of amplifiers. I don't know why. 7) Isobaric means two identical drivers = 2x$. Is it too much to expect an isobaric sub to occupy a minimum of space and still reach 20Hz or lower? And are there any driver recommendations out there? So far, the modeling I've done keeps bottoming out at 30Hz. Any and all responses will be greatly appreciated. Thanks!