Most open dynamic driver headphones are bandwidth-limited in the bass. This is why many bassheads tend to prefer closed headphones, because they can maintain more of the pressure necessary to produce the lower frequencies with generally less distortion.
Progress has been made in this area on some recent open DD designs though. I haven’t had the chance to listen to them myself, but some new open designs appear to have pretty decent LF extension, based on the graphs. Planar magnetics are also generally very good performers in this particular area. Though many enthusiasts feel they lack the slam or punch of a good dynamic driver (which could be related to other factors than just bass extension). And some can also have limits to their LF excursion, as Resolve has mentioned.
This is an example of why distortion (and also EGD imo) measurements are relevant to a headphone’s sound quality, performance, and EQ-ability btw.
Modal/cone breakup would also be a limiting factor at the other end of the frequency range. Some (mainly cheaper) headphone drivers can’t maintain their shape and pistonic behavior as well when played more loudly at higher frequencies, which effectively limits their speed, extension, and bandwidth in the upper treble.
Your takeaway: EQ can give you some ability to tweak a headphone’s tonality and bandwidth. But it can’t defy the laws of physics and the limitations of physical designs and materials.