low end floorstander or high end bookshelf?

Would you lose anything without that dedicated midrange driver in the 683? Or can single drivers in bookshelf speakers sound pretty darn close?

I agree that it is a matter of Quality vs Quantity, I would always go with quality.

At one time I had a large room (24x13) and had my two Rel Strata's hooked up to LS3/5a's, even though I had my main speakers which were Kef Reference 2 (excellent speakers in their own right), it was musical bliss when it came to the Ls3/5a's!

YMMV

Floorstander AND a sub :)

Does it have to be B&W?
You may find a good compromise with high-quality floor standers at the price of the 805.

The 805 Diamond will not have the scale and big sound presentation of the larger 683 even though it is supplemented by a sub. If you listen to a lot of music with thumping bass lines or favor a large scale and deep bass presentation, you would be better off sticking with the 683. If you prefer more refinement and poise and an overall higher quality sound, the 805 Diamond will be a better choice(though with reduced scale and frequency response at the lower spectrum).

Apart from that you didn't mention the size of the room. In a small room the 805 Diamonds will be more appropriate than the 683s. These monitors also demand more from partnering equipment and are not as forgiving as the 683s.

As a separate note, just like Wslam I would choose quality over quantity anytime although the speaker may not be full-range. In my view the overall presentation is more important than ultimate deep bass performance.

Quantity vs Quality. Only you will know. Everyone will have their preference. I personally have always preferred Quality over Quantity.