basically, a setallite and sub combination are compermise at the beginning. if u can afford a floorstand, always go for floorstand.
connect lineout from preamp to sub line in, then connect high pass from sub to amp, connect amp to setallite speakers. basically, this set up has 2 advantages, 1. the amp won't need to ampify the full signal, only higher frequency needed for setallite. 2. use sub's analog crossover means the whole system stay analog. (preamp can be fully analog, without any bass management, cause "bass management is dealt with inside the sub)
685, 683 are both excellent speakers. u also need to consider what amp you are going to drive them. B&W speakers are not particular easy to drive. how about 684? it's a floorstand, cheaper than 683.