As others have suggested get a speaker with good off axis response.
Think of speakers as lights. Most designs are like a spot light. A few are designed with wide even dispersion that makes the off axis match the on axis - a flood light.
What to look for? Take a look at Stereophile frequency plots where they show the on and off axis response for various degrees. Look for a design that is even and smooth both on and off axis.
Example of good "flood light" response
Example of Bad "spot light Response
Notice how the bad response had a huge hole in the midrange off axis. This means that with this design you will only get an even response when sitting close to the speakers and in the sweetspot.
Think of speakers as lights. Most designs are like a spot light. A few are designed with wide even dispersion that makes the off axis match the on axis - a flood light.
What to look for? Take a look at Stereophile frequency plots where they show the on and off axis response for various degrees. Look for a design that is even and smooth both on and off axis.
Example of good "flood light" response
Example of Bad "spot light Response
Notice how the bad response had a huge hole in the midrange off axis. This means that with this design you will only get an even response when sitting close to the speakers and in the sweetspot.