I'm in a similar situation to you, in that my listening room is also my living/dining room. Also there is an open staircase going up the front wall above my audio racks, so i'm very limited in what room treatments I can use. Fortunately the acoustics in my room are basically good. Though being able to place my racks a bit back from my speakers on a landing, leaving nothing between the speakers helps the sound staging. I have my speakers placed along the long wall about 2/3rds to the right, obeying the basic Cardas room setup principle of not placing the speakers dead centre along the long wall. There is also plenty of space either side of the speakers which helps.

Other things which have helped -

- Placing a medium size rug in front of my speakers as I've got tiled floors
- Having a large 3 meter long soft-apolostered sofa against the back wall
- Opting for timber in lieu of aluminium venetial blinds on my large floor to ceiling windows.

In terms of room treatment options - given my room limitations, I like Franck Tchang's Acoustic Resonators which are small, yet very effective & Synergistic Research Acoustic Art resonators. The SR resonators are a bit bigger, but are almost like works of art. Both are passive devices. Here are a couple of links fyi -
http://www.6moons.com/audioreviews/francktchang/resonators.html
https://www.synergisticresearch.com/acoustics/passive/acoustic-art/

With passive resonators, the best product is the one which you can get the best setup advice with. If I could get Franck Tcahng to personally treat my room whilst travelling to my neck of the woods, I'd jump at the chance.

Btw GIK Acoustics are a good value room acoustics company & offer high quality, yet reasonably priced acoustic panels (including art panels) backed by good advice/support. I have no relationship with them. They're my 'bang for the buck' pick - http://www.gikacoustics.com/

Remember proper room setup is key. There are a couple of room setup guides which i've found to be a good help. The Cardas Room setup guide is a basic guide to get you started. If you want to get into more detail, the 6 moons article is a more advanced guide -

http://www.cardas.com/room_setup_main.php
http://www.6moons.com/audioreviews/theroom/3.html

Hope that helps!