Loudspeakers with a wide sweet spot

A used pair of Avalon Acoustic Eidolons would fit the bill. Best off-axis response of any speaker I've heard.
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Any OHM Walsh speaker (largely omnidirectional).

Also Gallo Ref. 3s.

Blue Circle Pennies also. These are designed to go closer to walls.

Also some Dueval, Morrisons and a few other lower priced omnis.

Audiokinesis speakers offer a wide sweet spot if you have enough space to set them up in your room and use the recommemded 45 degree toe-in so the signas cross about 2' in front of of the listening position. Both the Jazz Modules and Dream Makers are in your budget.

Hmmm--I don't agree with the planar suggestion. Planar's can throw of a wide soundstage but the sweet spot is usually very small.

In general achieving a wide sweet spot I find comes at the price of imaging and detail. While the Eidolons do have a very good measured off axis response and sound decent off axis, they are far far better when seated exactly in between with very very precise set up.

My old Klipsch La Scala's had a wide sweet spot. Not great imaging or sound stage depth, but you could sit in many locations and get about the same presentation.

Personally at that price point I think B&W has a pretty reasonable compromise. It's not the best at imaging or soundstage depth but it's pretty darn good. It has very good off axis response and the listening experience off axis is very good, not nearly as hampered as many other speakers.

I do strongly recommend that you listen for yourself to some of the suggestions because it really is a balance. You'll gain a larger sweetspot at the expense of something else.