It IS possible to get a sealed box subwoofer with an F3 below 40hz, but you're right it will take a driver larger than 10". Parts Express has several high end subwoofer drivers in their "reference series" and "Titanic" lines in the 12" and 15" sizes that will reach F3's in the 30hz region, even lower with a Linkwitz transform LPF. I guess you have seen this one, http://sound.westhost.com/project71.htm.
Parts express even goes one better with a subwoofer designed for infinite baffle use. ( http://www.parts-express.com/pe/showdetl.cfm?Partnumber=295-455 ). This guy has a HUGE Vas of 8.73 ft3, along with an FS of 22hz. With a QTS of .59 this driver is designed to use your attic or basement as a sealed enclosure and should really rock the house.
My dream design for a subwoofer was to take two of their 12" reference series drivers in an isobaric sealed box and use the Linkwitz transform LPF to get the F3 down to 20hz. Elliott has circuits for very high power amps on his site too. Of course by the time I factor in the price for all the parts, the wood for the box, and my time, the ECS-10 might begin to look inexpensive.