Determining the frequency response of an audio crossover (something seems off)
I ran the circuit through Proteus simulator and the frequency sweep gave me these results:

NO LOAD:



6 OHM LOAD:



Both graphs run from 1k to 20k, and the key figures are as follows:

            fo         Go        f(-3)        Gpb        G(3.5)
NoLoad    5.9 kHz    - 3.3 dB    4.6 kHz    - 4.4 dB    -15.0 dB
Load      5.0 kHz    -10.0 dB    4.2 kHz    -12.4 dB    -20.9 dB

Where:
    fo     = Peak frequency
    Go     = Gain @ f0
    f(-3)  = -3dB point
    Gpb    = Passband gain
    G(3.5) = Gain @ 3.5kHz (cut off frequency of tweeter, from manufacturer)


For the LOAD graph, the 'passband gain' is read at about 8.1kHz; after this point it climbs back to -10dB at 20k.

CONCLUSION:

Check both your maths and your soldering.

I say the above with all due respect - I'm too lazy to do the maths these days so I'm definitely not one to judge. However, it does apear that you've 'got your wires crossed' in the assembly process and that there's some breakdown with the maths/MATLAB combo.

Check ground connections, and perhaps consider laying out the components better than that horrible spider-web monster on page 5 of the supplied datasheet!