It also depends on how sharp the dip is, if you put the mic about a foot from the speaker does it flatten out?
Don't expect to get a flat response in the low end range; +-6 to 8db is common.
The main thing is, if you can't correct it, is your aware of it...
Also try this, find the same frequency on a piano, or an open E on a guitar, and see if you can hear the dip...
Pure sine waves are the acid test, real music contains harmonics.
Not saying sines waves are worthless.
And yes speaker position is very important, your "room" is a big part of your monitoring..