B&W 685's Vs KEF LS50

I would go KEF R300

The ls50 would be an improvement in refinenement and tones,  but you may end up trading one 685 good quality for a ls50 good quality.   Like mzk says, you would get more out of the r300.

The Peachtree Nova 300 output with either of the speakers you mentioned I would never had considered.
The Nova 300 has an 300Wpc into 8 ohms (24.8dBW), 450Wpc into 4 ohms? Heck of a source powering speakers that you have or are considering. I’ve always been a B&W user, with that Nova 300 I wouldn’t have considered anything less than the 703 S2’s or something comparable to their specs at a minimum..

Wanted to add to my previous post: prices today and what you’re trying to do factors in most. Budget wise, I would look for a high sensitivity speaker setup to run with that Peachtree if you’re not into loud in your face type of music. Less sensitivity speakers require less power whereas high sensitivity require more power. *ie, 150-200 watt 95db speaker with that Peachtree would sound awesome with a small to medium listening room, you don’t need new or the best. Only thing you have to remember not to get alcohol ears and crank it up...*laffs

I apreciate all the feedback, just to clarify, I had the B&W 685s before the Nova 300 and realize they are small for the WPC of the Nova 300 but the clarity and sound stage is remarkable, especially with a decent sub in my medium sized room. I'm trying to stay under 2k if I upgrade but could be talked up to 3k. 
B&W 700 S2 or something like it is on my radar but I have heard remarkable things about the KEF LS50s and was curious.
As Chris R said it's probably a case of too close to be considered a real upgrade