One thing I would do is toe-in the speakers so they are facing the listening position. I would try to get the speakers equidistant from the back and side-walls, so the acoustic reflections hit you evenly. Just doing those two things can 'upgrade' the sound and cost nothing.
Before throwing a lot of money at new speakers, you need to know what is going on with your room. You should instead buy a measurement microphone and take some measurements of the frequency response at your listening position. This will tell you how to proceed next much better than asking a bunch of anonymous people on a message board. Once you have identified any problems, then you can attack them specifically. I would get something like
this, hook it up to a laptop and run some sweeps. After that, come back here and show us what you measured, then we can help you identify potential 'opportunities for improvement'.