I have these speakers myself. A friend and I took a deeper look at these speakers and did measurements. At first the graph looked liked yours, after measuring the tweeter and woofer separately we saw the crossover should be adequate for the driver design. In the datasheet of the speakers is also an almost flat response, which is weird (see picture)
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We tried flipping the polarity of the woofer around and were blown away. Now the measurements were matching the datasheet and the dip at 1.5k was gone. Could there have been a mistake in the design and the + and - swapped? After reinstalling in my setup I heard a lot more bass, my subwoofer and speakers were also out of phase. The sub is a JBL controll sb-5 so it is passive, didn't think of the polarity before. If this was a mistake in the design at tannoy I think its a major flaw. Maybe if you still have the speakers, you can retry the measurements and come to the same conclusion?