Well he said this: "The reason I keep coming back to it, however, is my 'bi-modal' experience of the DCA's, whereby sometimes they're kind of tame, and other times, they're powerful and dynamic.".I am not so sure I fully understand your analysis, although what you say is true, but the OP was talking on the relation with perceived Slam vs listening level, and if I am getting right you seem to talk about seal? Me I think it's simply a very inefficient headphone, with a very low impedance, It doesn't appear unrealistic that this driver can struggle a bit to reproduce the bass at low voltage, depending what amp is used to drive them if it's current limited. Add to that the general theory of Fletcher Munson curves, that are true for all headphones but this add to the effect of perceived anemic bass at low level it's just already a fact that you won't get the perception of Big Bass at low level, but this may even be enhanced by unneficients drivers, It may just need more to get to their fully linear dynamic behaviour. @JanesJr1 What amp are you using?
If they're behaving unpredictably from one listening session to the next then it could be seal / position related, as well as psychological (the burn-in I mentioned). I expanded on the point to explain that "slam" was about frequency response received at your ear, and ways in which to get a more reliable headphone experience - by choosing a headphone with low unit to unit variation and one that generally seals well to most people - as these variables will all effect the ability to get that "slam" & of course that reliability will also increase the accuracy in the rest of the frequency range too when it comes to using EQ's published on the interwebs.
EDIT: it's true that listening volume level will influence how much "slam" you perceive, with increased volume increasing the likelihood of experiencing it as the bass is brought out more, but if you get the frequency response right from 200Hz down to 20Hz then you don't need to listen at loud levels to get it. For me the Harman Curve bass allows for slam at normal listening levels.
Regarding amps, the mantra I'm most familiar with is that "if it gets loud enough then the amp is fine", as in you don't need extra "overhead" or whatever people sometimes talk about when justifying overpowered amps. I don't know if here on ASR an amp has ever been measured to show that it's fine at all frequencies & output levels but taps out when it has to play a bass note?