...Is there a device that will change the frequency response of a system to match this at normal listening levels? ...
Natural acoustic instruments have a different timbre at different volume levels, due not only to the non-linear response of our hearing being discussed here, but also being played louder or software changes the instrument's frequency structure or timbre. It seems that a loudness curve would have to know:
1. The absolute level at which the music is being played (what SPL would a full scale amplitude be)
in addition to knowing
2. The relative level at which the music is at every moment in time (something like a 3 second running average RMS amplitude).
The loudness curve should apply only to the first, not to the second, else it would squash or mask the timbre changes with musical dynamics that is an essential and natural part of the music.
Regarding (1), you would have to graph your system's SPL at listener position versus volume control knob position (e.g. preamp voltage), in order to tune the loudness contour to the SPL. This of course depends on the room size, speaker efficiency, and power amp gain.
PS: on 2nd read what I said sounds confusing. So here's the point: any loudness curve should be based on the actual SPL level of the loudest musical peaks (or average levels). If the music is dynamic and reaches a quiet spot, or a loud spot, the loudness curve should not compensate for that. This is natural and the change in timbre or FR is part of the musical event. Once calibrated the curve should depend on volume position only.