I'm no sub expert but you do realize that 12, 15 inch, etc and other drivers would have to move alot of excursion just to move same amount of air as 24 inch can? So while yours are moving only a little I'm sure you are not listening at reference or at least have it calibrated reference levels? In my opinion those 24 inch vids that are moving alot of air are probably playing 125db or more and probably not ideal in normal listening environments.
When you say high playback levels, what db are you referring to? Even though the driver's excursion isn't moving much as you suspect do you still get plenty of bass to your liking or do you feel like its not loud enough and missing bass?
Others who have build DIY boxes and such can probably give additional feedback whether its the driver, amp, or box itself.
Main theater: Sony 45es, 120 inch 16:9 screen, Panny BDT500, Rotel RMB-1075, Rotel RSP-1068, Klipsch RP-280F, RP-450C, RP-160M, SVS PB13 x 2, Monster Power Conditioner, GIK acoustic panels
TV Room: Panasonic 60" plasma, Integra DTR-30.3, Axiom Audio M60, VP150, QS8, EP500