First of all, we see this dip in virtually every system we install without exception - (yours too, sorry). It is referred to as the Allison Effect named after the great Cambridge, Massachusetts speaker designer Roy Allison who made the observation in the '70s. The Carl Tatz Design (CTD) professional monitor interpretation states that when a pair of monitors are set up on stands at ear level (tweeter ear height) behind a console, which is typically about 48"-50" off the floor, there is a low frequency cancelation, due to what is known as the 'floor/ceiling bounce" in the 100Hz - 150Hz range causing a deep and wide dip.
This "Grand Canyon" of missing low frequency information, often 12dB - 15dB deep, is THE primary reason everyone has such a difficult time with the low end during a mix. This makes it obvious why. So by example, if the track you're working on for instance happens to be in the key of this dip, say A, then the tonic notes are going to be weak, yet as you see in the graph above there is a peak centered around 65Hz which will encompass the D note (72Hz) which will cause that note to be hot, further exacerbating the dip in A.
See if this sounds like your experience with your monitors: Let's just say you're new to the room or the monitors and you haven't "learned" them yet. So you get the bass and kick to really pump just the way you want it to sound and then you take it out to the car and you have WAY too much low end because of the EQ boosts and level you used back in your control room. Soooooo, you go back to your control room and do the "learn-your-monitors-dance'' where you say to yourself, oh that low end sounds great - I better turn it down..... And so, begins your life of low frequency rationing - a sonic tragedy. But wait, there's more, suppose your track is in the key of D relative to the graph above and if so, you're likely to have the opposite experience out in the car because you ducked that frequency. So again, in this room, you may think that you've learned your monitors and have trained yourself to back off on the low end (even though you don't want to) when you find yourself scratching your head because now you have to go back to the control room to boost the bass. It can happen, not as reliably as the dip, but in this case the peak compounded the problem.
I'll bet that for most of you, no one has brought this to your attention before - and for good reason. The speaker manufactures know about it but aren't going to wave it in front of your face too much because there is no technology that is going to boost 15 dB at 100 Hz without blowing your near-field monitor's woofers if played at even normal 80dB - 90dB levels and certainly not 100dB and above. There is a solution to this that I'll tell you about in the next episode.