What could be instead of side walls?

All else equal, I'd move the speakers closer together so they are only about 5-6' apart if your listening position is only 5' from the speaker plane. It's good to start from an equalateral triangle and then tweak from there. Also start with the speakers tilted towards you. Again, experiment from that initial position.

As a next step, you may want to check the overall reverb characteristics. How far is the listening position from the rear wall? How far are the speakers from the front wall? Depending on that, you might be able to do some relatively inexpensive experimentation with side panels (reflecting vs. diffusing or absorbing) to see if that tightens things up a bit. A lumber yard (plywood sheets) and a couple of friends could do a lot here for purposes of seeing if that helps without spending much money. Lots of ways to go, but I'd move the speakers closer together first. Good luck. Let us know your progress.

Zaikesman, I was stretching a bit on that second point. Depending on the back wall and front wall distances, the side panels might make a positive difference (would have to work out all the ratios), but I'm with you and would do that as a last resort. Marakanetz, move the speakers as close together as you can w/o messing up your TV -- every little bit helps. By the way, I'm assuming you don't have the TV or anything else in between the speakers -- that would, as I'm sure you know, also mess up the soundstage. Good luck.