Welcome to my world (13' x 13' home office with all my electronics). Your space is a little bigger, but here's what I've learned:
1 - Overall the single best thing you could do to reduce your speakers' interface with the room (boundary effect, nulls & voids, all that) is to use sealed, acoustic suspension speakers). No other factor solves as many problems as that.
2 - That advice applies even if you're going to put them 3 ft. from the back wall or whatever space allows. It's simply better to not have a port pumping out low frequency sound into the room.
3 - After that comes the usual balancing act between a restrained use of sound abosorption panels w/ or w/o diffusion panels. May not be needed at all if the furnishings do the trick, carpeting, wall handings, and so on.
I can strongly recommend ATC sealed/acoustic suspension speakers. The passive SCM19 can be had new for ~$3,500, and used for considerably less. These speakers get rave reviews from reviewers and users alike. If you buy used, you might be able to get the active version of the SCM19; some people say they sound better. Personally, I always want to choose my amp, so go for passives.
I had the studio 2-way version (SCM12 Pro) for several years and thought the world of that sound: very dynamics, authoritative, detailed, yet not fatiguing or edgy. Those passives sounded great at very low volumes, something sealed passives seem to do better than ported. I use my office speakers (currently the vintage KEF 103.2 two-ways) with a sealed subwoofer & electronic crossover. Great sound.
