The B&Ws are a good choice and will easily fill a room twice the size of yours. B&Ws due tend to be a little pricier than others of similar sound quality. I tend to recommend speakers that have a similar driver configuration as the 684s for systems involving a separate sub. Other speaker companies’ products you may want to consider are; Aperion Audio, Ascend Acoustics, Athena Tech, Axiom, Dali, Definitive Technology, Dynaudio, Energy, HSU, Infinity, JBL, KEF, Klipsch, Martin Logan, Monitor Audio, NHT, Paradigm, Phase Technology, Polk Audio, PSB, RBH, Rocket, SVS, Vandersteen, Vienna Acoustics, etc.
What was your opinion of the KEF sub? Most HT enthusiasts will want a sub that digs deeper than the KEF. For a sub, you’ll want to look at HSU, SVS, AV123, eD, and Epic for starter, which are all internet direct companies.