Let's see if I can explain this right. Lets say you have a long excursion 15" woofer operating in a range of 50 to 1Khz with flat response. Then you drop the frequency down to say 25hz well the IMD will go up in the frequency range of 500-1Khz because the lower frequency will start modulating the upper octaves of that operating range. Hence higher intermodulation distortion from 500-1k.Simple explanation, it's like when you yell into a fan it modulates your voice.
Bill W.