notch in the low bass

[Williams Audio]

Hi,
well, i didn't become more enlighten from the 'time correction and units alignment' I launched a week ago; first i use a 3 way active Xover and may be? only the position of the units can make a change.
i didn't feel any differences by moving the position of the units.

What i did figure out is a notch in the low bass response, i use pink noise from a tdk cd, at about 80 hz is a notch, very audible.
The spk enclosure is 3cm thick MDF; h=98cm, w=27 cm, d=46cm, i use 2 scan-speak 8'' 8 omh
In parallel with a bass reflex port 8cm diameter/12 cm long.
Any ideas for testing and how to over come the notch are very well come.
Best regards
Williams