I recently spent a cuple of months with two subs for my Kef R100's. There was bass, no problem, and one could go much louder, no problem. I have measurement capabillities and aren't afraid of using an EQ (digital), lots of tweaking was done. But it never sounded right. Even if measurements didn't say so, it was always boooomy and uncoherent. I gave up.
Sub's where the bass sections of my JBL 4331B's (15" and very large), used full range they have stunning, fast and full bass.
Conclusion (not for the first time): If you want bass, use full range speakers. But feel free to experiment!
I suspect you may be lumping together a number of factors rather than separating them.
I too have some KEF coaxials but, like KEF themselves in their higher quality models, wouldn't consider using a 5" driver (equivalent to a 4" due to lost area for the tweeter) below about 400 Hz because not only does it have insufficient area for clean transients at standard levels the large cone deflections will lead to raised levels of distortion and more importantly modulate the coaxial tweeter output. High quality in the 80-400 Hz range requires adequate cone area in the main speakers. At a distance of 3-4 m in a room at standard levels this means the clean reproduction of around 112-115 dB transients (at 1m) and 4" drivers tend to be a good 10 dB shy of this. Close at a desk they may be OK(ish) but not as mains in a room. It requires a 10-12" driver or more commonly these days 2 x 7-8". This is what allows low frequency percussive sounds like drums to sound reasonably correct. Not "dynamic", "fast bass" or whatever but reasonably correct. It has nothing to do with subwoofers or low frequency extensions of the mains just cone area.
With a pair of subs and a pair of R100/LS50s the best approach isn't clear to me. Sitting the subs under the "tops" and crossing at 200-400 Hz to create mains with adequate cone area and then living with whatever careful EQ can do to improve the room response or crossing at 80-100 Hz and distributing the subs to improve the quality of the room response below 80 Hz and then living with the consequences of inadequate cone area in the mains. If the subs were capable of crossing high I suspect I would lean towards the former due to preferring to listen at standard levels and favouring clean percussion but would be a bit fidgety rather than happy and content. For those that tend to listen at quieter levels and with a different set of preferences it is easy to see a different setup being preferred. If the subs are individually DSP controlled and setup competently a preference for no subs makes little sense to me given they could be arranged to be effectively inaudible apart from absorbing some of the room boom. Not competently setup and emphasising the room boom then off may well be preferred.