Speakers with wide listening window

How about wireless earphones?

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.

Totem speakers are designed with very wide dispersion characteristics in both horizontal and vertical axes.

One of the larger models may do a good job in your [large!] room.

Try crossing the axis of the tweeters somewhere ahead of where you will be listening; the closer to the speakers they are crossed the broader the area of listening but as in most things expermentation is necessary. May help, if not you have no expense.

The "listening window" has two aspects: Tonal balance, and imaging.

In order to get good tonal balance across a wide area, you want a speaker whose off-axis response is very similar to its on-axis response. This also pays dividends in naturalness of timbre and reduced listening fatigue, as it's desirable for the reverberant energy in the room to have approximately the same spectral balance as the first-arrival sound.

In order to get decent imaging for off-centerline listeners, you want the far speaker to be louder than the near speaker to offset the difference in arrival times. This is harder to get, and requires well-controlled radiation patterns and proper setup (strong toe-in). The key is that the near speaker's output must fall off fairly rapidly and uniformly as you move off-axis towards the outside, so that the far speaker ends up being louder at least in the upper mid and treble frequencies, where we get most of our localization cues from. Imaging will still be best up and down the centerline, of course.

Imho, ime, ymmv, etc.

Duke
dealer/manufacturer