My advice is based on the idea that since this is a common space and needs to be dinner party friendly you cannot fully treat the room and solve all the core issues with panels. As such, I would look into merely "taming" the overall loudness, echo, and sloppiness of the overall room performance as opposed to focusing on specific issues related to stereo listening.
1st Suggested Approach
Put two vertical 2 x 4 panels at the same height as the TV, where the center of the TV's height is the center of the panels' height. You can then space them so they look balanced on the wall, filling in the bare wall space between the TV and the corners - but for artistic reasons I would not place them directly between the TV and the corners, but closer to the TV to take advantage of the horizon line aspect which will make the room look deeper in that direction. As in, place the panels at the 2/5ths point between the TV and the corners where they are closer to the TV. It will look like a larger room and be pleasing on the eyes.
Next, if you are up to it, I would consider putting foam corner block bass traps all along the ceiling / wall intersection above the TV. You can probably find a foam color which matched the paint of the trim in the room so it looks more like a soffit than an acoustic material until one looks very closely.
Those two things would help immensely.
2nd Suggested Approach
If you want to go more hardcore you could cover the entire wall with acoustic panels of a pleasing color making the whole wall appear to be an "accent wall" covered with fabric panels like you'd see in a hotel lobby or similar artsy public space. That would take more work, but you could do it fairly affordably.
3rd Approach
You could place 2 ft tall panels in a solid line centered at the center height of the TV across the entire front wall, making a lovely looking and effective reflection control line.
Regardless of which of those three you go with, or whatever you choose, think about adding acoustic absorbing materials to the room where you can. For instance, if you are hanging a photo, you could get or make a fabric wrapped fiberglass panel slightly larger than the photo frame and back it with a thin sheet of plywood which you run a bolt through for hanging the photo. Then mount the panel and hang the photo on the front of the panel. While the photo will still reflect treble, the midrange and upper bass characteristics of the acoustic fiberglass will be fully effective and taming the room. If you have an open shelf bookcase in the room, you could put 1" thick foam or fabric covered fiberglass against the room facing back of the shelf areas to absorb sound, place you books and trinkets in front of the foam/fiberglass. If you are hanging a stretched canvas artwork, fill the area behind the canvas with fiberglass or foam like I did (and posted photos on the forum) so the midrange is cleaned up a bit.
Basically, it is easy to make some effort to treat a room without ruining the design and aesthetics. In many cases the acoustic treatments can enhance the appearance of the room. I put some artsy looking diffusors / absorbers I got from GIK Acoustics in my family room and people love them. Not a single person who has seen them thinks they are unattractive.