To state "a 10" woofer will not keep up with a 6" midrange. Too much mass." is to oversimplify the situation. For one thing, different 10" woofers and 6" midrange drivers have differing amounts of mass; there are some 10" woofers with less moving mass than some 6" midrange drivers. But more importantly, the moving mass of any given driver is only one factor determining it’s "speed"---the size of the driver’s motor (magnet) is a huge factor. A driver with higher moving mass and a bigger motor can outperform a driver with lower moving mass but a smaller motor. By the way, the perceived "speed" of a driver is more a matter of how fast it stops moving when the signal does, and returns to "rest", than it is of how fast it starts moving. The cone of a dynamic driver not stopping when it should is called overshoot. As Newton’s Laws of Motion state, "A body in motion tends to stay in motion"---my own over-simplification!
Likewise, the statement "They must use very stiff suspensions to bring them back to neutral position after every excursion" is not universally true. The stiffness of a suspension is only one factor in the design of any driver, one needing to be balanced against other factors. Some very high performance woofers have stiff suspensions, some don’t. Acoustic Suspension designs (sealed enclosures) require suspensions with far less stiff suspensions than do Bass Reflex, for instance.
Woofers ARE often placed in MDF enclosures, but they don’t have to be, and sometimes aren’t. Baltic Birch plywood is a popular material used by some makers of high-performance subs. That plywood is far stiffer than MDF, it’s resulting resonance at a higher frequency than that of MDF, optimally far above the x/o frequency of the subwoofer.
"Massive objects resonate at frequencies that are hard, very hard to deal with". Sorry, also not necessarily the case. Internal bracing very easily and effectively deals with subwoofer enclosure-wall resonances, the braces pushing those resonances above any frequency the sub will be called upon to reproduce, reducing their audibility to below perceptible levels.
