sub in small room with bookshelf advice needed

My bookshelf speakers fill my small room well, however I am interested in adding a sub but uncertain if it will just cause boom. In other words, I am using bookshelf speakers due to the fact that larger speakers sound bloated...can I get a sub to cover the 20 to 40Hz gap and adjust in the gain to the point that it blends but does not boom?

Speakers: Usher BE-718
Room: 9x15x7 (WxLxH)...I know the Ushers are a little much as is for this room...

Also, what sub is quick enough to blend with the Ushers?

Thanks