In setting up speakers measure from the center of the drivers.
A rule of thumb is: many setups tend to end up with drivers located on a diagonal of the room. So it may be a good place to start. Draw an imaginary line from corner to corner of room, and notice, as you pull the speakers into the room they get closer together. Or conversly, as they move closer to the wall behind, they get further apart.
But unless the room is square, the side distance is never equal to the distance of the wall behind. This aids in spreading room modes so that fewer stack upon each other creating high pressure zone anomolies for specific frequencies.
Include the ideal critera of a unilateral triangle of 8'. That is, listener to speaker = 8', and speaker to speaker also = 8'. At least for critial listening.
However the width of my room is only 12', so to get some distance from side walls I have to use 7'. That puts the center of the drivers 30" from side walls: considered a minimum.
When the speakers are only a couple feet from side walls, try a device at the reflection location from speaker to lstener. I tape a mirror on a side wall, move it around until the image of the speaker appears in the mirror from the listening position. That is the spot. You can then use smallest possible devices since it is so accuratly located. A 2' square should suffice, and no danger of deadening the room.
Experiment with degrees of absorbtion of the device relative to the sense of space in the soundstage (use what is handy while experimenting: a pillow against the wall on a table, folded blanket, towels, a piece of clothing on a hanger, etc.). Or when there is a an abundance of absorbive pieces in the room, like much overstuffed furniture, etc., you may want to use diffusive devices instead: like a shelving unit. Anything to diffract the direct reflection from driver to listener, while not deadening the room further.
Coordinate this with toe-in of the speakers. I have found with box speakers (the worst design for room acoustics) a toe in of about 7 degrees is a good balance: avoiding exessive direct side reflections with width of soundstage (space). Pointing them directly at listener will offer the greater focus, but at the expense of space.
The issue with direct reflections of sound from side walls, floor, and ceiling, is the sound energy arrives slightly delayed than the direct sound from the drivers.
It can be more practical to treat the side walls than the floor or ceiling. However it is usually convenient to place a throw rug on floors without carpeting to aid the process.
With full range speaker, the best results in setup can be obtained by locating the woofers according to: the LINKS link at http://www.linkwitzlab.com
scroll down to 'Subwoofers: Optimum number and location', then to 'Getting the bass right' sublink.
Real results can be obtained from these efforts that will far outweigh any from top end gear, or cables and tweaks (which can be marginal at best, to non existant at worst).