Based on your room size, the 684 should work fine, especially when you don't mention any boomy bass problem. How did the Infinity speakers sound in your room? - If the 684 is better than the Infinity, then you should not regret your purchase.
However, it is true that a room with better acoustics can make an identical system sound much better. Since you cannot transport your friend's living room to your house, then you will just have to experiment with some room treatment as well as speaker positioning in your room.
A few other things to consider:
- Have the 684 been run in properly? They can sound a bit bright when not fully run-in.
- What cables are you using? If the brightness is bothering you, try to go for some warmer sounding cables, which usually tends to be ones without silver in the conductors.
- While your room length and width is not that small, your ceiling is a little low so I think a rug on the floor will help.
- Do not toe-in the speakers too directly at the listening position as that will also make the sound more bright.