It seems like the conversation is going in cricles and jumping across a variety of subjects as usual. Sensitivity, maximum output, limiters, test signals versus complex real content, etc. I'm going to repeat a lot of things previously mentioned here.
Sensitivity in the deep bass is largely
(but not totally) determined by box volume. HIL etc. I don't think anyone is arguing against that. I don't remember anyone being confused about this either so I'm not sure why it was brought up to begin with?
However I would like to point out that there can be some differences as I noted in some measurements posted earlier in the thread so I am quoting that post below because sensitivity was brought up again. There can be differences of 3 to 4 dB or more and this is not seen in WinISD or other modeling programs and the reason is partly because the data used for the simulations is incomplete and missing the complex inductance which is a major component in the final performance that can impact the response shape, efficiency and sensitivity in a big way which in turn affects the amount of power or voltage to driver excursion relationship. I've posted about this before and that is why I keep bringing it up. A lot of people consider inductance relatively unimportant for sub drivers operated <100Hz. After seeing how the real world result differs radically from the model due to the inductance being unnaccounted for in high inductance drivers, I consider that missing information pretty important. Albeit for a slightly different reason than the classic view on it. So yes at the end of the day there can be sensitivity and efficiency differences. Don't forget about varying impedance that makes it very difficult to match power input into 2 differing systems at any particular frequency too. This doesn't mean that HIL isn't true, it just means that it is a bit more complex than X size driver in X size volume will always have X sensitivity at X deep bass frequency such as something like WinISD shows. The programs that model this behavior are rare, but the worst part is that you simply can't get the information needed to do so on most large bass drivers anyway. Essentially you have to use something that has already been vetted by someone else, or use your best judgment and pick a driver and then test these things yourself. I put much less stock in simulation bench racing now than I did a few years ago.
Sensitivity is great but it is only one parameter. Deep bass is usually the first region of response to run out of headroom. Deep bass requires high displacement for high output especially using sealed enclosures. This is why there has been an explosion in long xmax drivers over the last decade and a half. Even the pro audio high efficiency drivers are recognizing the need for increased displacement and are sacrificing sensitivity and efficiency to some extent for power handling and xmax. In general the more sensitive the driver the lighter the moving mass and higher the motor strength. Both of which usually mean a shorter xmax or greatly increased cost. Longer stroke usually means a heavier moving assembly and reduced peak motor strength so efficiency goes down. It's a sliding scale. In the middle is a good region of overlap where you can get a reasonable amount of both or optimize for a certain application.
The above factors all reinforce the point that Bosso and also Mark are making that yes...There is quite a bit more to doing it right than stuffing any old driver in a sealed box and strapping an amp to it. A system as used typically involves the speaker, enclosure, amplifier, equalization, front end electronics, speakers being integrated with the subwoofer system and the room.