In a small room, a little absorption goes a long ways. This is because, within a given time span, a sound wave goes through many more reflections in a small room than in a big one. So, any little thing you do will have an effect.
YOu have two problems to deal with - the voicing of the room, and dealing with early reflections to get good imaging. For the latter, basically you treat the first reflections points on the side walls, floor, ceiling, and rear wall. Whether you use absorption or diffusion depends on which is best for the overall room voicing.
To get an idea of the voicing of your room, do two things: First, walk around the room clapping your hands or snapping your fingers, and see if there's a ringggg. IF so, you have high frequencies bouncing back and forth between two reflective surfaces, and you must treat one of them. The second thing to do is just walk around the room talking to yourself. Listen to the sound of your voice. Then walk through other rooms, talking and/or clapping. This will give you a feel for what various rooms sound like. If possible, do this in a room that you know sounds good - perhaps at a dealer's, or in a friend's house. The idea is to give you a feel for what a good room sounds like, so you can move in the right direction when you go to change the voicing of your room.
It sounds to me like you'll need a combination of absorption and diffusion. Large leafy fake plants are good for diffusion, and so are bookcases with books. For absorption, since it's a dedicated room, you can make moveable wooden frames say a foot wide and maybe four or five feet tall, and glue acoustic foam to the frames. Try leaning the panels up against the wall in various places - the first sidewall reflection points would be a good place to start.
The corners of the room are places where a little treatment goes a long way. Take care not to overdamp your room, as that's easy to do. But you don't necessarily have to spend big bucks to alter the tonal balance of your room - you can toss pillows into some corners, back issues of Sterephile or Listener or whatever in others, and so on. A stuffed listening chair might sound better than a wooden one, or vice versa. Generally speaking, natural fibers (such as wool or cotton) sound better than synthetics - keep this in mind if you're buying a rug.
My point is, you can significantly alter the voicing of your room, but you have to listen first (using your own voice) so that you know what you're trying to do.
Best of luck to you!