I'll start by saying I do something similar. I have Sonus Faber Venere 1.5s 3-4 feet apart, and I listen maybe 2 feet away sitting at my desk. I absolutely love having a wall of sound so close to me. People will tell you "Get those things on stands, spread them out..." blah blah blah...but if you're listening to these and enjoying it, then they're obviously sounding good to you.
If the Sierra 1's @ $850 are out of your price range, I'm not sure you're going to do better than what you've got. I'm a big fan of near-field listening, and I've found the sweetspot for me in terms of performance/price is the $1200 mark. If you can't hit that mark, or wouldn't want to wait for something used, I've been very happy with $400-$550 powered monitors (again, I don't think even those would do be any better than your 685's, especially in terms of quality of the higher frequencies.) I generally recommend anyone starting from scratch to give powered monitors a try. You already have other components to run traditional bookshelf speakers, so this doesn't apply.
Of course this is just what I prefer in terms of what sound I get for the money I have to spend. I started with some Def Tech StudioMonitor 450's, and I thought the sound of the Aon 3's from Goldenear was worth the extra money. My current Sonus Faber Venere 1.5's are worth every penny to me for near-field listening.
It's always easy to think there might be something better out there. Most people get something, but never really sit down and enjoy it. As a test, go audition some bookshelf speakers that are double the price of the 685's. Make sure they set them up like you would have them at home, at least in terms of spacing. Then sit as close as you would to them if you had them in your current setup. Do they sound THAT much better?