Single driver designs and imaging
In my listening history, one pair of small single driver speakers imaged so well they blew away all that came before. They are Omega TS2Rs, a model with 6.5 inch Fostex drivers that came out around 2002. At 94dB/watt/meter efficiency they are not difficult to drive. I first heard them powered by a small tube amp and later by a First Watt F2J. (The imaging was similar but the bass response was extended much lower with the latter, being a transconductance design. Everyone who has heard the TS2R/F2J combination asked about what subwoofer was being used. None was, of course.)
Sorry to digress, but I needed to establish that when properly driven this small speaker has none of the weaknesses normally associated with single driver speakers. The imaging is far beyond anything else I have experienced, excepting my own Hammer Dynamics Super 12s, which are in the same class but cannot replicate the ultra pinpoint nature of the Omega's imaging. Close though. Both share the ability to make one swear there are instruments four feet outboard of either speaker; soundstage is excellent. I have since gone to a bunch of big audio shows and have yet to hear anything remotely close to those little Omegas in imaging and rarely in the soundstage department. (This year in Chicago I did experience a million dollar class system that soundstaged brilliantly in a huge room, but that is what it took.)
For the well-heeled I can recommend an equally rewarding but different experience, the MBL Radialstrahler 101 E MK II speakers set up in a good room. Their omnidirectional presentation is perfectly executed and the sound field is so thrillingly immersive you forget all about the usual imaging impressions and considerations. I think they are about forty grand US a pair. The Europeans who report to my friend Dave (the TS2R owner btw) say the big shows over there have never had the MBLs set up well and it makes a huge difference, negatively. I have found through repeated listenings that the larger MBL system at $277K a pair has no advantage in a room of typical living room dimensions. In fact, I liked the smaller system better. Unlike the single drivers, where the sweet spot is rewarded, the MBLs don't care where you sit, or even if you sit. The experience is not like listening to a pair of speakers; it is a you-are-there feeling.
A boring TS2R audiophile review but it has pictures of the mighty mites. What might look like a tweeter is a port.
https://www.tnt-audio.com/casse/omegats2r_e.html