1) The SD-2 line will accept the same enclosures as the SA line -- they are slightly more sealed friendly. You can run the SA line sealed as well and many people do -- you more commonly ready about their usage as ported SPL subs on the forums but that is not all they can be used for.
2) The SD-2 line exists to fit much shallower enclosures (and they are slightly cheaper). Example... SD-2 10" is 4.75" deep and the SA-10 is 6" deep. The SD-2 also has a solid pole so does not need ANY room behind the magnet for venting making effective depth requirements even less.
3) The E, SD-2, and SA lines are made in China -- this has never been hidden. We have worked hard to ensure quality and hand test every one before it ships and pull out any units that don't meet QC standards. They would not be any better of a speaker if we built them here.
4) The SD-2 motor diameter is the same as the SA line -- it's just half as much height on the magnet. Not a small slug by any means.
5) As for ruggedness... they won't disappoint... the SA is obviously still a bit more rugged but the SD-2 holds its own :