A used pair of Avalon Acoustic Eidolons would fit the bill. Best off-axis response of any speaker I've heard.
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.
"In general achieving a wide sweet spot I find comes at the price of imaging and detail"
Hmm, I'm not sure that is necessarily the case.
I do agree about planar designs though, Maggies for example. The sweet spot is very small.
Better omnis I've heard, like OHM and mbl, certainly do not have imaging and detail shortcomings in their sweet spot however the sweet spot is much bigger and amenable to listening from various locations in the room than other designs, in particular when it comes to imaging and detail.
Omnis I've heard are way different though in how they do imaging. But I find their imaging is just different than conventional designs, not inherently better or worse in general. Some will like it and some not, same as with any good piece of gear.
Most speakers have a midrange scoop, by this I mean a dip in the off axis response through the midrange. If you want something with a wide sweetspot then you need something with an even off axis response. In a two way this is not easy to find because large woofers "beam". A three way with a small roughly 3 inch midrange like the Accuton on the Eidolon will be a good choice. Omni's will also work if you can pull them far enough out into the room so that rear wall reflections do not have any deleterious imaging effects.
Here is an example of a rare two way that would work B&W N805.
Regarding the Eidolons, I've lived with a pair for over 8 years and I'll stick by my recommendation for someone looking for speakers "with a wide sweet spot." Members of our local audio group certainly have no problem enjoying a listening session seated all across the room, and my wife, who is my regular listening partner, often enjoys sitting well off to one side enjoying the music we're playing. Yes, the holographic soundstaging is even better sitting right in the center, but the frequency balance, resolution and detail when listening well outside that center position is still exceptionally good.
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..
"Omni's will also work if you can pull them far enough out into the room so that rear wall reflections do not have any deleterious imaging effects."
Good point. This is key!
Blue Circle Pennies are supposedly designed to go closer to walls.
OHM Walsh speakers are acoustically attentuated in the wall facing directions so this helps them go a bit closer (two to three feet I would say in some cases involving smaller rooms) than otherwise with true omnis like MBL for example. For larger rooms, you will generally want them out further. My biggest room is ~ 30 feet deep and the OHM 5's in that room are a good 48 inches or more out from the rear wall. Soundstage is huge (wall to wall for many recordings) and imaging is sharp and focused in most of the room situated about 6 feet or further back from the speaks or so.
Mirage OMD-28. Reasonable size and sensitivity, fast, bass down into the 20's, very good imaging, turns the whole room into a good listening area. $7500/pair. When Chris Martens reviewed 'em for Absolute Sound, he bought the review pair.
I have their little brother, the OMD-15 and couldn't be happier with a speaker purchase, and they're my tenth or eleventh.
Although I am a big fan of horns and field coils, the speaker with the widest sweet spot that I have seen/heard so far is made by High Emotion Audio. The tweeter is unique and patented. It goes down to 2KHz- well within the range of image definition. It radiates nearly as well to the sides as it does forward. You can easily hear the soundstage image while standing off to one side, not even between the speaker. They are also not critical of setup- the image just grabs hold even if very casually placed.
This is one of the more high-speed, detailed speakers made, while being relaxed- an uncommon combination. The speaker (S7) is easy to drive with tubes or transistors. The retail is about $5,000/pair.
The tweeter is unique and patented. It goes down to 2KHz- well within the range of image definition.
This reminds me of the Energy Pro 22 - they had a hyperdome tweeter that they made themselves and which was crossed over at 1.8 Khz. Great speakers with a lovely natural wide dispersion and very reasonably priced back in their heyday. No doubt Dr. Floyd Toole had some influence on them. Of course, crossing over a tweeter that low has its problems in terms of maximum SPL without distortion and compression and potential to "blow up" but for a bookshelf the Pro 22's were outstanding sound for their modest price.
The speakers I own (Sforzando JL-1s), and other speakers designed by Fr. Richard Davidson (the old ITC speakers from way back, for example), would meet your requirements. You'd need me to get you in touch with him, though, as these speakers are not really commercially available or something you could listen to other than at a few people's homes. Generally I agree with the suggestions about omnis, and I have heard Avalons off-axis sound quite good, though I must say that they really can lock-in when you're in the right spot. One other more or less conventional speaker that I have heard and believe was not mentioned above that seems to do well from off axis is the von Schweikert VR4; I'd imagine that the entire VR line probably would be similar.