Polk SDA's if you don't mind buying used.
SDA's are no longer made but periodically can be found on ebay. They have a cord going speaker to speaker which feeds some right channel to left and left channel to right. The horizontaly opposed drivers partially cancel some of the left channel that reaches your right ear and right channel that reaches your left ear creating a wide sound stage. Kind of a headphone effect. With a good recording, I can space the speakers 6 feet apart, sit 8 feet back and get a 10 foot wide soundstage. Instruments appears outside the speaker spacing, not just at or between.
I guess if you have a 20 foot wide room you can just separate traditional speakers farther and be good to go. In a narrower room SDA's will expand beyond the walls. In situations where you can't place the speakers far apart, you will get sound beyond the speakers.
Used prices, approximate:
SDA CRS and CRS+ around $275-$300
SDA 2B around $300-$350
SDA1C around $400-$500
My SDA 2B's are over 20 years old and still sound great. IMO I haven't found better imaging for under $1400/pr in newer speakers, not even newer Polk RTi's or LSi's.
I use the SDA's for 2 channel stereo and in another room RTi8's, RTi6's and CSi5 for HT (Home theater).
P.S. I also liked Infinity alpha 40's. I imagine the Beta's would be good also. You can get these for $500-600 on the web.
I have not auditioned the speakers recommended by the others but I hope I get the chance.