You could try Ohm Walsh speakers or Decware ERRs. You could get the ERRs new within your budget and the Onkyo could drive them. The Ohms would probably work better with a more powerful, higher-current amp.
In either case, you'd get a wide soundstage from either of those speakers. The vertical plane may be a bit of a challenge.
In either case, you'd get a wide soundstage from either of those speakers. The vertical plane may be a bit of a challenge.