In need of some advice on a matching a sub

Wasn't worried about the crossovers more the tone of bass as the Kefs already have 6 subwoofers 4 10s in the front and 2 8s in the rears I don't want a tonal difference from the sub to the speakers

I still want the bass from the speakers tho, they have subwoofers in them and they work well at the moment it's just for that extra oomf when an explosion hits really 

yeah I did think that I’ve currently got my rears and fronts set to large on the settings, the majority of my use is movies but I do on occasion listen to music and as I’m looking to spend a big chunk on a sub was wondering if there was anything out there that would do the best of both worlds, I contacted kef and asked about the 209 subwoofer but I just can’t warrant spending £6k on a subwoofer, I’ve also been advised against goofing for that sub as there is another one on the market and you can get 2 for the same price and with those two it produces a better end product than the single kef I never use to have this issue I used to use he model 4s for stereo using a nad 600w per channel power amp and a hitachi pre amp (both old I know my dads old stuff) I bi wired the two fronts and that made a big difference just not enough wondering if I hoocked the nad up to the dennonif that would make the fronts produce more bass then I could bi wire the rears instead