Connecting a subwoofer to the line out connectors on the KN5000 does not disable the internal speakers. If you wish to disable them the simplest way is to plug in a set of headphones.
Reccommending a suitable subwoofer is much more difficult. It is highly dependent on personal taste, type of music to be played and the environment in which it is to be used. The subwoofer must have its own power amp either inbuilt or separate. As a general rule bigger and heavier can have the capability to produce better results. It is one those cases where size does matter but there are small units which may satisfy your requirements as well as larger units that are absolute rubbish. If you give more details of your requirements perhaps someone can make a specific recommendation. From my point of view I haven't yet found an inexpensive smallish unit that satisfies my ears and so when I take the KN7000 out I generally use it stand alone unless the venue has a suitable system to which I can connect.

Cheers
Len C.