Help, tweeter's frequencies dips

Hi All,
First of all, I am a beginner in diy speaker building.
Currently, I am confused regarding my tweeter's measurement result that showing big dip in freq between 5 khz- 10 khz. This happens to both tweeters. What am I missing in this measurement? Can anyone help me, please?

Tweeters are SB Acoustics SB26STC 4 ohm mounted into speaker box with baffle of 23 cm x 60 cm. Tweeter is "almost" flush mounted due to my miscalculation of router's depth, the flange still 1mm protruding from baffle:(
http://www.sbacoustics.com/index.php...le/-/view/400/

Tools:
- Muse class D amp, M20 EX, 20 watts into 4 ohms to tweeter.
- REW software
- UMIK-1 measurement usb microphone
- a laptop to run REW, give pink noise, connect also to Umik-1.

Thank you in advance,

Pacogun

Attached Images

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