OK... maybe it's a 3.5db dip from peak to trough... in any case it's far from a flat line despite being produced by a single driver with nominal phase swings in the same range. Although I suppose it could be more an issue of the cabinet resonances creating peaks... not sure. In any case, it's not horrible by any means - there are TONS of speakers that look far worse... but there are plenty that look much better too.
For the most part (and to my definition) the mid-bass region starts about 140Hz and ends around 400Hz... it's where lots of exciting stuff happens - second only to midrange - at least that's my take on it. Of course, someone else might only like picolos...
Although I've seen variances in definition that put mid-bass as high as 600-700Hz and others that narrow it further and inclued regions between 80Hz - 600Hz as being LoBass,MidBass,HighBass...
And of course everyone's favorite around this forum... SubBass - or ~80Hz and below.