The frequency range we hear the least but tend to love the most - Subs. Some consider these to be 120Hz and below but in my opinion Sub-harmonics don't kick in 'til around 80Hz, where an acoustic kick drum often produces tone. Dance or synthesized kicks are another matter and essentially extend down to 60Hz, tailing off at 50. These are immensely heavy frequencies, requiring most of your power to drive them and as such demand absolute control by an engineer. Awareness of the Sub-Harmonic curve needs to be taken into account here, for there are further frequencies extending down to 20Hz and below, which need to be addressed. In order to allow 60Hz to "speak" clearly the bandwidth extending to 30 (an Octave below) has to be rolled-off. More often it's what we take out as opposed to what we put in and in the range of subs it couldn't be more true. I roll-off my entire mix during the first stage of mastering at 50Hz, essentially a Butterworth curve at approx 24dB per Octave. Throughout the EQ stage in mastering there is also a secondary HPF (High Pass Filter - "everything higher shall pass") set at 40Hz with an even steeper curve. This ensures that I am not boosting wide bandwidths of subs in order to create that wonderful thump, rather specifically targeted areas so as not to cloud or smear this frequency range. If you're not using as least one main HPF across your mix you're asking your compressor to attempt to apply gain reduction to "the sound of the Planet turning." Roll them off and I am sure you'll be astounded by the increase in clarity and definition in your sub harmonic range.