The impulse response is a transfer function. Changing the measurement level doesn't change the transfer function until the system reaches either its saturation threshold at the high end or the noise floor at the low end. Within the linear range of the system measuring at different levels produces the same transfer function and so the same frequency response. Presenting the result in that way is quite counter-intuitive for people, however, so REW offsets the frequency response to account for the measurement level.
For sweep measurements the distortion graph shows how the transfer function's harmonic distortion relates to its linear response (labelled fundamental). That fundamental is the same as the frequency response shown on the SPL & Phase graph, perhaps barring cal file effects depending on the preference setting for whether distortion results include cal.
For FSAF measurements the situation is quite different. Any stimulus is allowed, and the stimulus may have more energy at some frequencies than others, important information to interpret the TD+N result, which is the difference between the system's total output and its linear output. The fundamental on the distortion graph shows the power spectrum of the linear part of the system's response to the stimulus. To maintain some commonality with the presentation of sweep results, that spectrum is shown at the SPL corresponding to the measurement level. For example, if white noise were the stimulus and the noise was playing at 80 dB SPL the power spectrum would sit at 80 dB SPL. That isn't the true spectrum of course, since 80 dB is the total SPL of the signal, the spectrum of that signal from a 64k FFT would sit at about 35 dB SPL. That would, again, be rather counter-intuitive, hence the chosen presentation.
. Compare all the cabinets, and wonder if the cause of differences is due to change in driver excursion, air velocity, turbulence, rasonant panels, leakage, etc. Just don't forget to stop and enjoy the music once in a while...
