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.

The T27 has a peak at 10 kHz as well as dropof above that when crossed over by a 18 dB filter, KEF then made a more advanced one.

So this is what it looks when I connect crossover built as this. I use a 2.5 mH coil and low resistance in series with the bass driver.  All caps are plastic not electrolytics.

This is the result for the tweeter section, how it will work I have to measure.

This is the woofer and woofer plus crossover, there is a minor rise in system Q from 0.95 to 1.05

The work will be to match the woofer and tweeter, I am pretty confident that the woofer section will work, but not so sure that the tweeter section will tame the T27. Time will tell.