But how are you level matching things? The input sensitivity of the SA1000 may be lower compared to the Kef amp. In reality, if both subs were to be getting 2.83V at the woofer terminals, the dayton might have 3db on the Kef even at 100hz (where the difference is largest). That would move the whole line way up showing that the Dayton kills the Kef, except at high frequencies then the battle is a little closer, which is what I'd expect.
I'd suggest doing the same measurements with the same amp. And not because of the low pass, that's all taken care of in your receiver. Keep that at 200hz if you really want to know the high frequency performance of the Daytons.