It seems like the k 53 has more negative reactance or more chamber area than the K-52. I am suspecting that when the k701 used in conjunction with it has a higher efficiency, narrower bandwidth, or just a higher center frequency and its low end is being extended with the higher chamber volume. Even just to hit a 700hz crossover while remaining flat.
A k 33 15" woofer tends to be a couple db higher in efficiency than the k 22 when used as a direct radiator. I would therefore expect that 2 to 3 db less attenuation would be needed for a k601 to match it, provided it was maintaining the same overall efficiency as the 701. The motors might also be magnetized to different tesla values. I can do a spectrum analysis on both drivers each using both horns. But I dont have an anechoic chamber. How ever a difference should be notable