Looking at the data and diagrams on the Neumann site suggests the kh750 doesn’t add a lot to the kh310 in terms of max SPL at 50Hz or below at 3% THD. the KH310 is a big speaker, and the KH750 is a small subwoofer, so it’s not surprising they’d be close in bass performance.
Adding subs will help even out bass modes in a room, and you get 6dB more bass by adding them if you don’t high-pass the KH310s, so that could be a benefit. If you have space and want really deep bass, you would want bigger subs than the KH750.