There have been a number of suggestions made in this thread, from replace your subwoofer, get two subwoofers, replace your B&W speakers, get a more powerful amplifier, to get a different house. All of these cost a lot of money and may or may not solve the problem. Before we recommend that you throw a lot more money at this problem, let’s at least discuss the various things you can try that will cost nothing, only some of your time and effort.
First of all, in your original post you said, “This is not working, I think the speakers are either too small or just don't work for my particular room.” Can you help us understand what you think is lacking? Is it that the overall sound is just too small? Or is it something else?
Because this is your first home theater, it is possible that you may have made a mistake or omission during the setup of the receiver – something that may be obvious to us but not to you. Did you read and follow the steps in the owners’ manual for setting up your Denon receiver? I know that Denon is infamous for poorly written manuals that are confusing. This also includes how you blended the subwoofer with the main speakers. This isn’t simple, and you do have a lot of options that will require some trial and error.
I remember helping a neighbor set up his HT a few years ago. He did all the basic set up and wiring, and I came over later with my SPL meter to help him balance the sound. Something sounded wrong to me, and it took a while before I asked to see just how he wired everything. It turns out that he used a pair of RCA audio interconnects between his DVD player and the receiver, just as he had always done with standard analog stereo gear in the past. He ignored using a digital connection between the DVD player and the receiver, and of course couldn’t get true 5-channel sound. I’m not saying that you’ve made that particular mistake, only that you may have skipped something important that you didn't think would matter.
Finally try rearranging the furniture. Don’t give up on this until you try it. Your wife may actually like it better once she sees what you have in mind. Your room is large and the 16 feet between the sofa and the TV and front 3 speakers may be too far. Before you move anything, first try listening closer to the speakers, about 8-10 feet away. Is that any better?
Then try putting the TV and front 3 speakers where your sofa now is and put the sofa with its back toward the kitchen. Leave plenty of room behind the sofa to allow easy passage between the hallway and the kitchen. This may solve at least two problems. It will shorten the distance between the listeners and the speakers. This will also allow you to spread the B&W 684s further apart from each other creating a wider sound. Finally, once you also relocate your subwoofer, you may find that it sounds much better than before.