I used omnimic to measure how the KEF 104aB filter worked with my drivers, unless stated otherwise distance was 50cm and on axis of tweeter.

The high pass filter section of the filter works well but the boost above 10 kHz is notto be found.

With regard to the woofer section it looks as expected, as measured on axis of woofer.

Adding the two resulted in a severe dip in response- 10-15 dB, black curve
Reversing phase on tweeter reduced the dip by more than 5 dB brown curve
Removing the L2 (0.6 mH) reduces the dip further, green curve.
I also tried removing the parallell cap to get a 6dB filter, and while it eliminatess the 2.5-3 kHz dip it also lowers the 500 Hz to 2 kHz range to much

 Measuring at 1 m gives similar results, with the dip and a too hot T27
The T27 is not flush mounted, edges are not rounded and there are some large bolt heads sticking up that holds the woofer so some of the peaks and dips really is not the drivers

I will have to take a look on how the competition in my collection of drivers measure up compared to the T27.